I think this depends on use and soil.
Not sure where you live, where I live, my LK3054 with loader and rops weighs in at approximately 3800-4000lbs with fuel and all, and 5foot is perfect.
It's really a weight thing, if you are trying to rip through rock infested soil like me, you need lots of weight.
Would think a 4 footer about right for a 20 in these conditions.
If grading and moving soil, I would think 5 footer on your machine would be fine.
But ripping hard soil I'd think you a little to light for anything more than 4 foot.
Good luck,
Joel