Just for reference in case anybody else is looking, here's what a 4-year-old looks like inside that seat. He just turned four a few months ago and is slightly short for his age, although I don't know the exact height. The harness straps work, I wish they came in the seat a little bit lower but I don't think he's going to have a problem. I adjusted them a little bit after taking this picture so the chest strap is a little lower.
The only issue I see in my case is that I don't think he's going to be able to comfortably wear a helmet while strapped in like this. I am mostly okay with that, as I don't believe a helmet is necessarily required while strapped in the way he is. But there's a few places I was hoping to go that require helmets. I'm going to try it on him and see how it works, but I'm not very optimistic at this point.