Bob,
My rear tires are, If I remember correctly, 43x16x20's.
I had the valve stem at 12:00 (AM or PM - it don't matter

)
When I asked here on TBN how much they thought it'd take - An educated guess was about 50 gal. I did jack the tractor up fairly high to tilt the wheel back away from the valve stem, which may have helped with the extra gallonage.
I just filled them until the air-release valve would only release fluid, indicating that the level had been reached.
I used Zecol Blast WW Fluid. It was on sale for $1.39 per gal at the local Farm & Fleet. It was supposed to be the -25 stuff, but surprise-surprise, the online ad was wrong - It was -20 rated. They wouldn't knock anything off though, and as a businessman, I cannot stand the folks who cry "False Advertising" at every turn, so I accepted the deal, as it was still about 50 cents cheaper than other -20 stuff, and was an actual name-brand product to boot.
I'm confident it will do just fine...
How many hours on your 4720?
Has it been a good ownership experience thus far?
Thanks, Lunk