I first bought the Adams model from REI when my son was 3.5. He was a bit young for it, could barely reach the pedals, got antsy back there, similar to what others have written.
Regarding the Adams bike itself --terrible clamp setup. It actually broke on us at the beach last summer and my son fell over while we were riding. Took him awhile to get over that. That's where REI's return policy came in handy. We traded in the Adams for REI's own Novara Afterburner, which has a much more sensible and robust clamp.
I did purchase a seat back for my son, and that has worked out beautifully. When he was smaller it gave him the option of letting go of the handlebars and I had extra peace of mind that he was strapped in. I suppose it's the kind of accessory you could get lots of places, but I found mine online at Sunrise Cyclery on Long Island.
www.sunrisecyclery.com