In addition, I was very pleased with the team's tireless effort in pursuit of rent collections, which improved to 91% in the fourth quarter.
Following the payoff of our remaining $100 million balance on our revolver, we have over $100 million of cash and a fully undrawn $350 million revolver and are now positioned to take full advantage of our unique and valuable partnership with GIC and opportunistically execute on our external growth plans.
Since our acquisition, Tesla announced a new $1.1 billion assembly plant.
And Barshop & Oles announced plans for a new $1 billion mixed-use development directly across the highway from our property.
This quarter, we signed a total of 120,000 square feet, up 10% year-over-year.
At under $11.50 of ABR per square foot, we also see a solid mark-to-market opportunity upon release of these spaces.
We ended the fourth quarter with a signed but not open backlog of $3.2 million, up from $3 million last quarter.
We are currently tracking roughly $2 million of ABR that is currently in lease negotiation, up from about $1.5 million last quarter, giving us some visibility on offsets to potential future fallout.
These 11 projects consist of redemising, expanding or combining spaces similar to the 18 targeted remerchandising opportunities that we completed in 2019.
94% of our portfolio by ABR remains open, unchanged from last quarter, with 4% of our closures tied to our theaters.
With that in mind, we established a $0.075 per share common dividend for the first quarter of 2021.
For the fourth quarter, our bad debt and abatements were $4.4 million, about the same as the third quarter.
We reserved about 89% of our uncollected fourth quarter recurring billings, leaving limited downside from these categories relative to our fourth quarter run rate.
As of year-end, $18.1 million of recurring billings for the period of April through December 2020 remain outstanding, of which $11.9 million have been reserved.
We expect most of the unreserved amount of $6.2 million to be paid back over the course of 2021 and 2022.
We continue to be quite pleased with the resiliency of our portfolio and our limited bankruptcy exposure as our leased rate of 92.8% held up well in the quarter, down just 50 basis points sequentially, primarily driven by the recapture of our two Stein Mart leases that I noted last quarter.
Blended rent spreads for the quarter remained positive at 3.4%, impacted by a few flat strategic renewals.
The leases that we signed during the quarter, annual contractual rent increases, were about 150 basis points, which is a key contributor to creating a long-term sustainable NOI growth profile.
And as Brian noted in his remarks regarding dividend policy, we have placed a premium on free cash flow, allowing us to utilize our cheapest form of capital to take advantage of these opportunities as we restabilize our portfolio to pre-COVID leased rate levels of nearly 95%.
Couple this distinction with an increased confidence in our business and in our access to low-cost debt capital, we no longer felt the need to maintain an outsized cash balance and repaid the remaining revolver balance of $100 million last week.
We ended the fourth quarter with trailing 12-month net debt to pro forma adjusted EBITDA of 7.4 times, up slightly from 7.2 times last quarter as another COVID-impacted quarter entered the calculation.
We remain committed to bringing leverage into our long-term target range of 5.5 to 6.5 times and expect to see steady improvement in EBITDA as we return to more normalized reserve levels and as our signed leasing backlog begins to kick in over the course of 2021.
We are establishing 2021 operating FFO per share guidance of $0.77 to $0.87, which is an expected improvement over the annualized 2020 fourth quarter operating FFO per share of $0.72.
Key drivers from there are a favorable impact of $0.03 from interest expense, primarily due to the repayment of our revolving line of credit, and $0.02 from our signed not open backlog that we expect the vast majority to open ratably in 2021.
While our assumption is that these circumstances will improve over the course of '21, we felt establishing a wider range was appropriate given the potential varying outcomes for our theater exposure, which represent about $0.10 per share of earnings.
Also, while acquisitions are not assumed in our guidance range, we intend to redeploy $100 million of cash on the balance sheet into opportunistic acquisitions that meet our strict underwriting standards, representing upside to our range.
As part of our ongoing efforts to improve our disclosures, this quarter we added a net asset value page to our supplemental on page 14 to help facilitate your analysis of our real estate value.

Summary:
As of year-end, $18.1 million of recurring billings for the period of April through December 2020 remain outstanding, of which $11.9 million have been reserved.
We are establishing 2021 operating FFO per share guidance of $0.77 to $0.87, which is an expected improvement over the annualized 2020 fourth quarter operating FFO per share of $0.72.