craigslist a Specialized HotRock.
I loathe the company, but bought one for my youngest when she was about 4.5yo. 16" wheels fit her fine- she is in the 60th% for height. She rocks training wheels on it still as we use her older 12" bike as a balance bike for her to practice on from time to time.
$50 for a used one is a steal.
If you buy a used HotRock, you will have a high quality bike for him and can allocate the other $ towards a true balance bike which will get him riding without trainers a lot faster.
For what its worth, I wouldn't have my kid go near a bike that doesn't have a coaster brake, much less ride it, when she is 4. Hell when she is 5 even. Coasters exist for a very good reason- they are the easiest for new riders to use and lock the wheel the fastest.