I've never owned a Hydra-Cutter but I've read many, many reports about them from guys who have owned and used them for many years. So far, I've never heard a bad word about that mower. It does exactly what you expect a bush-hog mower to do.
The problem with the PTO from a 4 digit tractor is that it has no mounting bracket IF....you are talking about the one that is hidden below the seat and accessed from between your legs. If you are talking about the early 4 digit tractor with the PTO mounted BEHIND the seat, then that's different. IT will work just fine on a 3 digit model because it uses hoses, not steel lines.