I would buy a XR 50. My son is 4.5 and has a pw 50. He is big for his age at 50lbs but last time we were riding he swapped bikes with another kid on an xr 50 and he did great with the bike just left in second gear. He was a little tippy toe but since he has been riding over 1 year he did just fine. The other kid was almost 8 and he still fit on the XR 50.

My son also liked the foot brake on the xr50, he had no trouble working it. His pw 50 has 2 hand brakes and that was what held him back when he first started riding at 3, he could not properly apply the brakes so I had to keep him in a field and watch him close. he could ride, control throttle and balance excellent but just could not squeeze the brakes enough to stop on a big downhill.

I would also consider a PW 80, 3 speed auto and if he can touch/ pick it up much more bike to grow into. I have a pw 80 ready and waiting, my son just needs a few more inches. He will be on it in the next few months as it is not much bigger than an xr 50.

I would not consider any of the race bikes, polini, LEM, KTM, unless he already really knows what he is doing, and wants to get into racing.