I have a Novara (REI) model, and am planning on getting another, as our younger son has graduated from the trailer. I suspect the typical 20" models might be a bit big for a 4 year old, though.
My older son didn't like the trail-a-bike at first. He was already riding a two wheeler when he started, but it took him quite some time to get over the release of control of the bike. It didn't take long, but it was difficult at first to get him to even do a lap of the playground.
My younger son, who is five and has been riding a two wheeler since age four, took to it immediately, and he had no size issues with the Novara 20" model (and he's not big for his age).
I've had no complaints about the Novara, and truthfully, it's hard to find other trail-a-bikes in stock in the city.
Does your kid already ride a two wheeler?