4x4, any vehicle with 4 wheels and a drive system powering all 4
A pre-runner, the normal definition would be any vehicle used by a race team to inspect and pre run a race course. Any vehicle that is capable of covering the course. The vehicles are normally street legal, and designed to be comfortable for the driver.
If you want to build a rig in a desert racing style, it can be 2 or 4wd, anything you want. I would personally go with a nice long travel suspension, 4wd, to make sure you can get home.
The most important thing is that you have fun building, working on, and driving it. Other than that, it can be whatever yo want.