I think your tractor would probably handle a chipper that does up to 6" stuff.
Someone posted that the hp goes up by the square root of the log diameter. I chip up to 3 1/2" logs in my little MacKissic chipper and have NEVER EVER bogged it running off my 18.5 hp JD 4010, hydro.
It would certainly handle the 4 to 4 1/2" logs that a DR Chipper (made by MacKissic; some have said) does.