It has two 7 gallon tanks and I used just over a quarter of one of the tanks for 3.3 hours of mowing so far, my guess would be about a little less than a quarter of a tank for those 4 acres.
My 2001 ZD21 60" can mow my 2.9 acres 4 times without filling up. That is usually 1.45 to 2 hrs. The thing is a miser on fuel. The old woods 6190 with a 16 hp briggs vanguard would use 5 gal per mowing at about 2.5 hr. The ZD holds a little over 5 gal.