I'll be watching the responses too, I have an older gary fisher trail a bike that I got used but have never used. My son is 4 and can't reach the pedals yet. He also rides a bike with training wheels, but not fast enough to stay up with even a very slow rider like me, and he still gets very distracted by everything as we're riding. He doesn't like riding in a trailer, though, so we don't even take it out any more.
My concerns with the trail a bike is that it might be hard to handle if he shifts his weight- there is some play in the connection, so the bike can sort of shift side to side kind of like he does with the training wheels.
vickie