I found a model for 2-wheeled robots here:
How should I adapt it to a 4-wheeled setting?
If only two wheels are connected to motors (and the other two are just used as pivots) then you can use the same model (the wheels on the same side, either left or right, have the same velocity).
If you have motors on all four wheels then it's a bit different story, you should first think about why you would do that. The problem there arises if the wheels on the same side (either left or right) do not turn with the same angular speed (or speeds proportional to their radius if one is larger) as that would cause drifting which you would have to take into the account. If they do in fact turn with the same angular speeds then again, the 2-wheeled model applies to this system as well.
The easiest way to built a 4 wheeled robot with motors to all 4 wheels is to buy a Rock crawler off of the internet, and to remove it's body and Radio receiver.
Then connect it's motors' and steering motor's wires to the output of a microcontroller and you have your 4WD robot, with the differential n steering system with suspensions n all.
If you are building a differential drive robot, best way is to give power to 2 diagonally opposite wheels and keep other 2 wheels freely rotating. This means your robot's platform should be circular