I bought a King Kutter 4 foot tiller from TSC a number of years ago. Used it with a 22 HP JD sub-compact. I used mine strictly for working up the garden, but I've since quit using it as much. If I were working up ground that already had vegetation on it, I'd first burn it off with Roundup. Also, as far as the size goes, you should be fine with a 4 foot model. With a 21 HP tractor, I wouldn't want anything bigger. Those tillers can put a load on a tractor in certain soil conditions.
