[Ryan_Todd]

I'm surprised that nobody has mentioned this yet, but one of the biggest reasons to use mecanum wheels on a demo bot is for the shock-and-awe factor! In that regard, 4" or even 6" wheels are a bit small-- this makes it harder to see what's going on and how the wheels work.

I would personally suggest 8" wheels for this application, although 10" mecanum wheels aren't a bad choice either. AndyMark offers both 8" and 10" varieties, and I can personally recommend them for their durable construction-- although I will definitely admit that the VexPro mecanums are easier on the eyes.