Depends on where you live, if it's parked under trees, covered or in shade, etc. If you keep the 4Runner well-detailed with a good coat of wax, that will help with dust, minor scratches from things falling on it (hail, small tree branches, etc.) and tree sap, etc. As far as paint fade goes, that will depend on the paint itself (longevity) and how much sun beats on it. If you're not in a very rainy or snowy area, I wouldn't worry too much about rust. Most rust occurs on the undercarriage and results from water spraying up from driving on the road or mud and leaves getting stuck (and not cleaned out) of tight places like corners of the wheel wells. As long as you check to make sure the underside of the truck is clean every time you wash it, you should be able to avoid most corrosion.
I would recommend using a sun shade for protection of your dash and interior from UV, fading, and cracking.