I do have a few at home, just not to sell for a 4 year old. My recommendations are:
The Vaterra Slickrock is small, durable and fun. The price is a bit high but it does include everything you need. If looking for used also look for it's direct predecessor the Losi Mini Rock Crawler (Vaterra is owned by Losi and took over this model).
http://www.greathobbies.com/productinfo/?prod_id=VTR01013
If you want a little bigger 1/10th scale is the norm. The Axial AX10 used to be the one to get and was very popular and well supported, but I guess they stopped producing it. Most vendors are going for scale now but it's more expensive, but if you're will to pay the Axial SCX10 chassis is again very popular and well supported, especially if you want to have fun upgrading it.
An important consideration is local store support as it will break, and if you want to be able to get it up and running again quickly you'll need a store with parts. Scope out Dynamic Hobbies, Great Hobbies, or Discount Hobbies and see what they have.