@SteamLouis-- It might not be a worthwhile investment for you because your son has almost reached the age limit.
I bought a Trail a Bike for my daughter when she was four and she has been riding on it regularly since the past two years. She is six now and I think we will continue to use it for several more years. It was definitely a good investment for us. It has made family bike rides and camping trips easier and more fun.
The only downside is that it's important for the child to keep his or her hands on the handle. And the parent should not cycle to fast or take turns too fast or the child might fall. So although the equipment itself is reliable and sturdy, some safety precautions are necessary.