Mudbug, the top speed of the vehicle will depend on the engine and the tire size. A smaller 21" tire size with say a 14HP Briggs on a Max II wouldn't run as fast as my Max II with an 18HP Briggs (that has more torque to spin the tires at higher speed) and 22" tires. If your Max IV has 26" tires with a 25HP Kohler it would run faster than a Max IV with 22" tires. Taller tires gear your machine up quite a bit. You would be surprised how much your gear ratio changes even with tires that are only 1" larger or smaller. The best way to find out how fast your vehicle actually goes is to get a GPS.
If anyone wants to check out how different tires sizes affect gear ratios, take a look here:
DML Tire and Wheel Calculator
Anyways, back to the topic. I would say that I would certainly be happy with 22" tires like Rawhide III's. They fill the fenderwells up nicely, they look and perform well, and you won't risk the tires rubbing through the body.