A 5 footer would probabally be your upper limits. Not so much for HP, but for the size of your tractor, that would be a lot of weight hanging pretty far back. "Tail wagging the dog" is the anology that is used a lot for something like that.

If you do go with a 5 footer, it will probabally limit you to a lighter duty model.
If you drop it down to a 54", you can probabally get a little more robust cutter and be okay. 4' may not cover your tracks. I like a cutter to be at least as wide as my trackwidth. Do depending on your tire option and how they are set. You may want to take a measurment:thumbsup:
As for price, anywhere from $500 to $1500 new. Just depends on what brand/model you get. The $1500 cutters are built much better and will last longer. Brands like Woods, Landpride, Bushhog and the like come to mind. Closer to $500 cutters would be things like Countryline @ TSC, or KingKutter. They are a lower quality, but still a good cutter for lighter duty "homeowner" type work.
And then there is always used. And prices vary by location. Around my parts, decient older Bushhog brand cutters, and even some not-so-old KK's can be had for under $200:thumbsup: for 5' models. But the 48" and 54" used models still command a premium.