Total sales were $617 million in the quarter with sequential increases in June and July.
Compared with the prior year, sales were down 15%, which is consistent with the forecast we provided in early August.
Fourth quarter On-Road sales were down 44% from the prior year.
The U.S. is the largest portion of On-Road and it accounted for much of the decline as the cyclical slowdown in Class 8 truck production was magnified by the pandemic.
As a reminder, On-Road first-fit in the U.S. is only about 3% of total Donaldson sales so our aggregate exposure to that market is limited.
Sales in Off-Road were down 24% in the quarter.
On the other hand, Off-Road sales in China were up nearly 50% in the fourth quarter.
Fourth quarter aftermarket sales were down 11% reflecting a decline in the mid-teens for sales through our independent channel.
Sales of Aerospace and Defense were down 3% in fourth quarter driven by soft sales of products for commercial helicopters.
Turning to our Industrial segment, fourth quarter sales were down 15% driven in large part by the Dust Collection business within Industrial Filtration Solutions or IFS.
Fourth quarter sales of our Downflo Evolution dust collection systems were up in the low teens and the sales of those replacement parts grew more than 30%.
The Downflo family of products is only about 15% of total dust collection sales today, but it has grown rapidly as customers appreciate the space and energy savings it offers and we value the ability to retain the aftermarket.
Fourth quarter sales of Process Filtration were down in the low-single digits after an increase of more than 10% last year.
Sales of Special Applications were down 10% in fourth quarter.
Fourth quarter sales in Gas Turbine Systems or GTS were up 6% due primarily to strength in small turbines.
We delivered 20% in the fourth quarter and 18% for the full year.
I'll start with operating expenses, which declined 10% to $125 million in the fourth quarter, that's flat sequentially, and it's our lowest fourth quarter level in four years.
Fourth quarter gross margin was up 20 basis points in the prior year, and our full year rate was up 50 basis points despite headwinds in lower sales and higher depreciation related to our capacity expansion projects.
That's why our capital expenditures in fiscal '21 are planned well below the $122 million we invested last year.
Replacement parts now account for 64% of total sales giving us confidence in the durability of our business model.
Mix benefits and lower raw material costs after the loss of leverage results in a year-over-year margin increase of 20 basis points in the fourth quarter.
Moving back to the P&L, Other income was $2.7 million in the fourth quarter compared with an expense of $0.5 million in the prior year, and improved performance in our joint ventures was a benefit in fiscal '20 and the fourth quarter expense in the prior year reflects a charge related to our global cash optimization initiatives.
With that in mind, it's best to compare the reported fourth quarter tax rate of 21.1% with the prior year's adjusted tax rate of 21.4%.
While the delta between rates is not significant, I'll point out that current and prior year rates were well below what we would typically expect.
As we think about fiscal '21, we see our full year tax rate going up in 2020 to be more in line with our long-term estimate of 24% to 27%.
Our leverage ratio was 0.9 times net-debt-to-EBITDA and in the fourth quarter we paid off a term loan for $50 million and we reduced borrowings in our revolver by $110 million.
Our fourth quarter and full year 2020 cash conversion rates increased meaningfully to 165% and 103% respectively and we plan to exceed 100% again this year.
Sales are expected to vary widely by geography and market and sales of our replacement parts and products for new markets should continue to outperform the company average.
Additionally, we expect sales during the first half of '21 will be down versus the prior year due to the timing of when the pandemic began.
We are seeing these sales trends play out in August, which we expect will be down about 10% from the prior year.
Resetting our annual incentive compensation plan generates a headwind of about $13 million and we are planning to make further investments in our strategic growth businesses and technology development.
Finally, we plan to repurchase at least 1% of outstanding shares in fiscal '21, which would offset any dilution from stock-based compensation.
Should conditions improve, it is not unreasonable to assume this goal above the 1% in fiscal '21.
Sales of process filtration were about $15 million in fiscal '20, that's an increase of more than 60% over the past three years.

Summary:
While the delta between rates is not significant, I'll point out that current and prior year rates were well below what we would typically expect.
Sales are expected to vary widely by geography and market and sales of our replacement parts and products for new markets should continue to outperform the company average.
Additionally, we expect sales during the first half of '21 will be down versus the prior year due to the timing of when the pandemic began.
Finally, we plan to repurchase at least 1% of outstanding shares in fiscal '21, which would offset any dilution from stock-based compensation.
Should conditions improve, it is not unreasonable to assume this goal above the 1% in fiscal '21.