There are plenty of Jeeps and other 4WD vehicles on the DLS. By using an invisible road, you can have a 4WD vehicle drive on that over dirt.
For just 4WD vehicles on roads, you will need to clone a region and update the config.txt file to include or replace the carz listed with the ones you want. With TS12 Sp1 HF4 and up, you need to use track-object vehicles and not scenery objects on your roads. You might want to update the carrate value too while you're at it to reduce the number of vehicles on the road.