Thanks for your response...I bought the 990X yesterday. It's a stud of a mower!Your 4720 is rated at 56 PTO hp. The Woods RD 990X is rated for tractors in the 45-75 hp range and the PRD8400 is rated for tractors in the 25-45 range. Sounds like you have too much tractor for the 84" mower but could obviously use it if you were careful. The 90" mower is right in your hp range. As a comparison my neighbor has a 72" Woods finish mower that he pulls with a 29 pto hp tractor and has no problems at all with it.