Will have a look tuesday when I go in, I only do a few days or so here and there.
Is he a big 4 or small 4?
I find that 4/5 to be that difficult age to size them for bike, they are too big for the toy bikes but not quite big enough for something with a chain and a full set of brakes, stabilisers are a bit of a non starter, take the pedals off, even going so far as pulling the chain set and BB and then let them learn to scoot and balance.
Also consider a like-a-bike/balance bike depending on how weeny he is, maybe even a tagalong.