Parts availability has got to be on top of the list of stepping up to hobby grade. Looking at the Vaporizir you want a 4x4 truck; at least something that sits up high enough; with big tires; probably no smaller than 1/10 to be able to run in grass. I would ask yourself how proficient is your 4 year old at driving at higher speeds? But a lot of hobby grade RCs radios have EPA (end point adjustments) to allow you to set how far the speed control goes (limiting the speed). I would stick with a RTR (ready-to-run) well know brand Kyosho, or Tamyia only because of parts.

I don't know what type of buget range you have, and like SyCo_VenoM's signature says..."With great speed comes greater repair bills."

Just check for replacement parts and maybe find a RTR that has a charger/battery with it. Unless you plan to upgrade a lot.