Exhibit 99.4

SUPPLEMENTARY OIL AND GAS INFORMATION(UNAUDITED)

The disclosures contained in this
section provide oil and gas information in accordance with the U.S. standard, Extractive Industries Oil and Gas. Penn Wests financial reporting is prepared in accordance with International Financial Reporting Standards
(IFRS) as issued by the International Accounting Standards Board.

NET PROVED OIL AND NATURAL GAS RESERVES

Penn West engaged independent qualified reserve evaluators, GLJ Petroleum Consultants Ltd. (GLJ) and Sproule Associates
Ltd. (Sproule), to evaluate Penn Wests proved developed and proved undeveloped oil and natural gas reserves or to audit Penn Wests evaluation thereof. As at December 31, 2012, substantially all of Penn Wests oil
and natural gas reserves are located in Canada with less than one percent of our proved reserves located in the United States. The changes in our net proved reserve quantities are outlined below.

Net reserves include Penn Wests remaining working interest and royalty reserves, less all Crown, freehold, and overriding royalties and other
interests that are not owned by Penn West.

Proved reserves are those estimated quantities of crude oil, natural gas and natural gas liquids
that can be estimated with a high degree of certainty to be economically recoverable under existing economic and operating conditions.

Proved
developed reserves are those proved reserves that are expected to be recovered from existing wells and installed facilities or, if facilities have not been installed, that would involve a low expenditure to put the reserves on production. Proved
developed reserves may be subdivided into producing and non-producing.

Proved undeveloped reserves are those reserves that are expected to be
recovered from known accumulations where a significant expenditure is required to render them capable of production.

Penn West cautions users
of this information as the process of estimating crude oil and natural gas reserves is subject to a level of uncertainty. The reserves are based on economic and operating conditions; therefore, changes can be made to future assessments as a result
of a number of factors, which can include new technology, changing economic conditions and development activity.

YEAR ENDED DECEMBER 31,
2012

CONSTANT PRICES AND COSTS



|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|-------------------------------------------------------- |  | -------------------------- | --- | - | ---------------- |  | -- | --------------- |  |  | ------------------------- | - |  | ------------------------------- | -- | - |  |  | --- | -
|Net Proved Developed and
Proved Undeveloped Reserves
(1) |  | Light andMedium Oil(mmbbl) |     |   | Heavy Oil(mmbbl) |  |    | NaturalGas(bcf) |  |  | Natural GasLiquids(mmbbl) |   |  | Barrels of OilEquivalent(mmboe) |   
|December 31, 2011                                        |  |                            | 254 |   |                  |  | 47 |                 |  |  | 630                       |   |  |                                 | 20 |   |  |  | 427 |  
|Extensions & Discoveries                                 |  |                            | 5   |   |                  |  |   |                 |  |  | 15                        |   |  |                                 |   |   |  |  | 8   |  
|Improved Recovery                                        |  |                            | 21  |   |                  |  | 3  |                 |  |  | 11                        |   |  |                                 | 1  |   |  |  | 27  |  
|Technical Revisions                                      |  |                            |    |   |                  |  | 2  |                 |  |  | (20                       | ) |  |                                 | (1 | ) |  |  | (3  | )
|Acquisitions                                             |  |                            |    |   |                  |  |   |                 |  |  |                          |   |  |                                 |   |   |  |  |    |  
|Dispositions                                             |  |                            | (44 | ) |                  |  | (4 | )               |  |  | (9                        | ) |  |                                 | (1 | ) |  |  | (50 | )
|Production                                               |  |                            | (23 | ) |                  |  | (5 | )               |  |  | (101                      | ) |  |                                 | (3 | ) |  |  | (48 | )
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Change for the year                                      |  |                            | (41 | ) |                  |  | (4 | )               |  |  | (104                      | ) |  |                                 | (4 | ) |  |  | (66 | )
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|December 31, 2012                                        |  |                            | 214 |   |                  |  | 42 |                 |  |  | 526                       |   |  |                                 | 17 |   |  |  | 360 |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Developed                                                |  |                            | 148 |   |                  |  | 40 |                 |  |  | 463                       |   |  |                                 | 15 |   |  |  | 279 |  
|Undeveloped                                              |  |                            | 66  |   |                  |  | 2  |                 |  |  | 63                        |   |  |                                 | 2  |   |  |  | 81  |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Total(2)                                                 |  |                            | 214 |   |                  |  | 42 |                 |  |  | 526                       |   |  |                                 | 17 |   |  |  | 360 |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  



|(1) | Columns may not add due to rounding.
|--- | ------------------------------------

|(2) | Penn West does not file any estimates of total net proved crude oil or natural gas reserves with any U.S. federal authority or agency other than the SEC.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------



---

YEAR ENDED DECEMBER 31, 2011

CONSTANT PRICES AND COSTS



|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|-------------------------------------------------------- |  | -------------------------- | --- | - | ---------------- |  | -- | --------------- |  |  | ------------------------- | - |  | ------------------------------- | -- | - |  |  | --- | -
|Net Proved Developed and
Proved Undeveloped Reserves
(1) |  | Light andMedium Oil(mmbbl) |     |   | Heavy Oil(mmbbl) |  |    | NaturalGas(bcf) |  |  | Natural GasLiquids(mmbbl) |   |  | Barrels of OilEquivalent(mmboe) |   
|December 31, 2010                                        |  |                            | 232 |   |                  |  | 47 |                 |  |  | 685                       |   |  |                                 | 17 |   |  |  | 410 |  
|Extensions & Discoveries                                 |  |                            | 15  |   |                  |  |   |                 |  |  | 31                        |   |  |                                 | 2  |   |  |  | 22  |  
|Improved Recovery                                        |  |                            | 29  |   |                  |  | 1  |                 |  |  | 41                        |   |  |                                 | 3  |   |  |  | 40  |  
|Technical Revisions                                      |  |                            | 3   |   |                  |  | 4  |                 |  |  | (13                       | ) |  |                                 | 1  |   |  |  | 6   |  
|Acquisitions                                             |  |                            | 4   |   |                  |  |   |                 |  |  | 10                        |   |  |                                 |   |   |  |  | 6   |  
|Dispositions                                             |  |                            | (5  | ) |                  |  |   |                 |  |  | (14                       | ) |  |                                 |   |   |  |  | (8  | )
|Production                                               |  |                            | (22 | ) |                  |  | (6 | )               |  |  | (111                      | ) |  |                                 | (3 | ) |  |  | (49 | )
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Change for the year                                      |  |                            | 24  |   |                  |  | (1 | )               |  |  | (56                       | ) |  |                                 | 3  |   |  |  | 17  |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|December 31, 2011                                        |  |                            | 254 |   |                  |  | 47 |                 |  |  | 630                       |   |  |                                 | 20 |   |  |  | 427 |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Developed                                                |  |                            | 186 |   |                  |  | 45 |                 |  |  | 560                       |   |  |                                 | 17 |   |  |  | 341 |  
|Undeveloped                                              |  |                            | 68  |   |                  |  | 2  |                 |  |  | 70                        |   |  |                                 | 3  |   |  |  | 86  |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  
|Total(2)                                                 |  |                            | 254 |   |                  |  | 47 |                 |  |  | 630                       |   |  |                                 | 20 |   |  |  | 427 |  
|                                                         |  |                            |     |   |                  |  |    |                 |  |  |                           |   |  |                                 |    |   |  |  |     |  



|(1) | Columns may not add due to rounding.
|--- | ------------------------------------

|(2) | Penn West does not file any estimates of total net proved crude oil or natural gas reserves with any U.S. federal authority or agency other than the SEC.
|--- | ---------------------------------------------------------------------------------------------------------------------------------------------------------



|                                                 |  |      |        |   |      |   |        |  
|------------------------------------------------ |  | ---- | ------ | - | ---- | - | ------ | -
|CAPITALIZED COSTS                                |  |      |        |   |      |  
|As at December 31, ($CAD millions)               |  | 2012 |        |   | 2011 |  
|Proved oil and gas properties                    |  | $    | 19,798 |   |      | $ | 20,235 |  
|Unproved oil and gas properties                  |  |      | 609    |   |      |   | 418    |  
|                                                 |  |      |        |   |      |   |        |  
|Total capitalized costs                          |  |      | 20,407 |   |      |   | 20,653 |  
|Accumulated depletion and depreciation           |  |      | (8,906 | ) |      |   | (8,342 | )
|                                                 |  |      |        |   |      |   |        |  
|Net capitalized costs                            |  | $    | 11,501 |   |      | $ | 12,311 |  
|                                                 |  |      |        |   |      |   |        |  
|COSTS
INCURRED                                   |  |      |        |   |      |   |        |  
|For the years ended December 31, ($CAD millions) |  | 2012 |        |   | 2011 |  
|Property acquisition (disposition) costs(1)      |  |      |        |   |      |   |        |  
|Proved oil and gas propertiesacquisitions       |  | $    | 52     |   |      | $ | 138    |  
|Proved oil and gas propertiesdispositions       |  |      | (1,667 | ) |      |   | (404   | )
|Unproved oil and gas properties                  |  |      | 37     |   |      |   | 181    |  
|Exploration costs(2)                             |  |      | 241    |   |      |   | 330    |  
|Development costs(3)                             |  |      | 1,595  |   |      |   | 1,417  |  
|Joint venture, carried capital                   |  |      | (137   | ) |      |   | (107   | )
|                                                 |  |      |        |   |      |   |        |  
|Capital expenditures                             |  |      | 121    |   |      |   | 1,555  |  
|Corporate acquisitions                           |  |      |       |   |      |   | 286    |  
|                                                 |  |      |        |   |      |   |        |  
|Total expenditures                               |  | $    | 121    |   |      | $ | 1,841  |  
|                                                 |  |      |        |   |      |   |        |  



|(1) | Acquisitions are net of disposition of properties.
|--- | --------------------------------------------------

|(2) | Cost of geological and geophysical capital expenditures and costs on exploratory plays.
|--- | ---------------------------------------------------------------------------------------

|(3) | Includes equipping and facilities capital expenditures.
|--- | -------------------------------------------------------



---

STANDARDIZED MEASURE OF DISCOUNTED FUTURE NET CASH FLOWS AND CHANGES THEREIN

The standardized measure of discounted future net cash flows is based on estimates made or audited by GLJ and Sproule of net proved reserves. Future cash
inflows are computed based on constant prices and cost assumptions from annual future production of proved crude oil and natural gas reserves. Future development and production costs are based on constant price assumptions and assume the
continuation of existing economic conditions. Constant prices are calculated as the average of the first day prices of each month for the prior 12-month period calendar period. Deferred income taxes are calculated by applying statutory income tax
rates in effect at the end of the fiscal period. Penn West is currently not cash taxable. The standardized measure of discounted future net cash flows is computed using a 10 percent discount factor.

Penn West cautions users of this information that the discounted future net cash flows relating to proved oil and gas reserves are neither an indication
of the fair market value of our oil and gas properties, nor of the future net cash flows expected to be generated from such properties. The discounted future cash flows do not include the fair market value of exploratory properties and probable or
possible oil and gas reserves, nor is consideration given to the effect of anticipated future changes in crude oil and natural gas prices, development, asset retirement and production costs and possible changes to tax and royalty regulations. The
prescribed discount rate of 10 percent is arbitrary and may not reflect applicable future interest rates.



|                                                         |  |      |         |   |      |   |         |  
|-------------------------------------------------------- |  | ---- | ------- | - | ---- | - | ------- | -
|($CAD millions)                                          |  | 2012 |         |   | 2011 |  
|Future cash inflows                                      |  | $    | 23,141  |   |      | $ | 30,815  |  
|Future production costs                                  |  |      | (10,359 | ) |      |   | (12,287 | )
|Future development costs                                 |  |      | (2,372  | ) |      |   | (2,350  | )
|                                                         |  |      |         |   |      |   |         |  
|Undiscounted pre-tax cash flows                          |  |      | 10,410  |   |      |   | 16,178  |  
|Deferred income taxes(1)                                 |  |      | (1,242  | ) |      |   | (2,463  | )
|                                                         |  |      |         |   |      |   |         |  
|Future net cash flows                                    |  |      | 9,168   |   |      |   | 13,715  |  
|Less 10% annual discount factor                          |  |      | (4,054  | ) |      |   | (5,080  | )
|                                                         |  |      |         |   |      |   |         |  
|Standardized measure of discounted future net cash flows |  | $    | 5,114   |   |      | $ | 8,635   |  
|                                                         |  |      |         |   |      |   |         |  



|(1) | Penn West is currently not cash taxable.
|--- | ----------------------------------------



|                                                                         |  |      |        |   |      |   |        |  
|------------------------------------------------------------------------ |  | ---- | ------ | - | ---- | - | ------ | -
|($CAD millions)                                                          |  | 2012 |        |   | 2011 |  
|Estimated future net revenue at beginning of year                        |  | $    | 8,635  |   |      | $ | 6,081  |  
|Oil and gas sales during period net of production costs and royalties(1) |  |      | (1,640 | ) |      |   | (1,878 | )
|Changes due to prices and royalties related to forecastproduction(2)     |  |      | (1,865 | ) |      |   | 2,068  |  
|Development costs during the period(3)                                   |  |      | 1,736  |   |      |   | 1,821  |  
|Changes in forecast development costs(4)                                 |  |      | (1,387 | ) |      |   | (1,774 | )
|Changes resulting from extensions and improved recovery(5)               |  |      | 526    |   |      |   | 1,256  |  
|Changes resulting from acquisitions of reserves(5)                       |  |      | 2      |   |      |   | 115    |  
|Changes resulting from dispositions of reserves(5)                       |  |      | (1,221 | ) |      |   | (164   | )
|Accretion of discount(6)                                                 |  |      | 864    |   |      |   | 608    |  
|Net change in income tax(7)                                              |  |      | 636    |   |      |   | (917   | )
|Changes resulting from technical reserves revisions                      |  |      | (49    | ) |      |   | 118    |  
|All other changes(8)                                                     |  |      | (726   | ) |      |   | 1,300  |  
|                                                                         |  |      |        |   |      |   |        |  
|Estimated future net revenue at end of year                              |  | $    | 5,510  |   |      | $ | 8,635  |  
|                                                                         |  |      |        |   |      |   |        |  



|(1) | Company actual before income taxes, excluding general and administrative expenses.
|--- | ----------------------------------------------------------------------------------

|(2) | The impact of changes in prices and other economic factors on future net revenue.
|--- | ---------------------------------------------------------------------------------

|(3) | Actual capital expenditures relating to the exploration, development and production of oil and gas reserves.
|--- | ------------------------------------------------------------------------------------------------------------

|(4) | The change in forecast development costs.
|--- | -----------------------------------------

|(5) | End of period net present value of the related reserves.
|--- | --------------------------------------------------------

|(6) | Estimated as 10 percent of the beginning of period net present value.
|--- | ---------------------------------------------------------------------

|(7) | The difference between forecast income taxes at beginning of period and the actual taxes for the period plus forecast income taxes at the end of period.
|--- | --------------------------------------------------------------------------------------------------------------------------------------------------------

|(8) | Includes changes due to revised production profiles, development timing, operating costs, royalty rates and actual prices received versus forecast, etc.
|--- | --------------------------------------------------------------------------------------------------------------------------------------------------------