The main difference between the Sportsman 400 & the Sortsman 500 HO, is the "Overbored" engine with a different carburator & the EBS "Engine Brake System", which the 400 does NOT have. Otherwise the machines are Identical. According to the Specs, they even weigh the same.
I have both a 2001 Sportsman 500 HO & a 2002 Sportsman 400. My 400 had the "EBS" added as an Option. The only difference you will notice in the power is the Pull the 500 has when you "Punch It".
The EBS Option is nice though, especially in Hilly Terrain. My local dealer offers the EBS as an Option for $150.00, as long as you get it before you take delivery of the machine. He has a deal worked out with Polaris to send back the "Non EBS" parts back to Polaris for Credit.