In addition, we kept our full year adjusted EBITDA margin to within 50 basis points of 2019, despite significantly lower market demand, and we generated record free cash flow.
These factors help us narrow the gap in rental revenue year-over-year from being down over 13% in Q3 to down just 10% in Q4.
It was $275 million, almost 13% higher than prior year, and it was driven by healthy retail demand.
In total, we plan to open another 30 specialty cold starts this year, which is double the number that we opened last year.
And this will bring us close to 400 specialty locations by December.
Rental revenue for the fourth quarter was $1.85 billion, which was lower by $208 million or 10.1% year-over-year.
Within rental revenue, OER decreased $190 million or 10.9%.
In that, a 5.6% decline in the average size of the fleet was a $98 million headwind to revenue.
Inflation of 1.5% cost us another $25 million, and fleet productivity was down 3.8% or a $67 million impact.
Sequentially, fleet productivity improved by a healthy 420 basis points, mainly from better fleet absorption.
Rounding out the decline in rental revenue for the quarter was $18 million in lower ancillary and rerent revenues.
As I mentioned earlier, used equipment sales were stronger-than-expected in the quarter, coming in at $275 million.
That's an increase of $31 million or about 13% year-over-year, driven almost entirely by an increase in retail sales.
That reflected OEC sold up 35% year-over-year in the retail channel for the second quarter in a row.
Used margins in the quarter were solid at 42.5%.
Notably, these results in use reflect our selling over seven-year-old fleet at just shy of 50% of original cost.
Adjusted EBITDA for the quarter was just under $1.04 billion, a decline of $117 million or 10.1% year-over-year.
The dollar change includes a $143 million headwind from rental.
And in that, OER made up $140 million and ancillary and rerent together were the remaining $3 million.
Used sales were a tailwind to adjusted EBITDA of $11 million, which offset a $3 million headwind from our other non-rental lines of business.
And SG&A was another benefit in the quarter of $18 million, with the majority of that help coming from lower discretionary costs, mainly T&E.
Our adjusted EBITDA margin in the quarter was 45.5%, down 150 basis points year-over-year, and flow-through as reported was about 66%.
Adjusting for those two items implies a margin of 46.1% and flow-through for the quarter of just over 56%.
A quick comment on adjusted EPS, which was $5.04 That compares with $5.60 in Q4 last year.
For the quarter, rental capex was $176 million, bringing our full year spend to $961 million in gross rental capex, which was 55% less than what we spent in 2019.
Proceeds in 2020 from used equipment sales were $858 million, resulting in net capex of $103 million.
ROIC remained strong at year-end, coming in at 8.9%.
That continues to meaningfully exceed our weighted average cost of capital, which currently runs about 7%.
Year-over-year, ROIC was lower by 150 basis points, primarily due to the decline in revenue.
Turning to free cash flow, which was a record for us at over $2.4 billion in 2020.
This represents an increase of over $860 million versus 2019.
As we look at the balance sheet, our having dedicated the majority of our free cash flow to debt reduction in 2020 resulted in a $1.9 billion or almost 17% decrease year-over-year in net debt.
Leverage was 2.4 times at year-end, down from 2.6 times at the end of 2019.
We finished 2020 with just under $3.1 billion in total liquidity.
That's made up of ABL capacity of just over $2.7 billion and availability on our AR facility of $166 million.
We also had $202 million in cash.
We're planning for another strong year in used sales, and we'll look to increase our capex spend to replace that fleet.
Within our guidance, that reflects over $1.9 billion in replacement capex.
In 2021, we expect another year of generating significant free cash flow, and that's after considering a return to over $2 billion in capex spending.

Summary:
Adjusted EBITDA for the quarter was just under $1.04 billion, a decline of $117 million or 10.1% year-over-year.
We're planning for another strong year in used sales, and we'll look to increase our capex spend to replace that fleet.