[Ether]

Originally Posted by Bjenks548

Now if you drive the right wheel forward it pulls forward and left, and the left wheel backwards, backwards and left. Shouldn't you go left?

Here's where your upcoming physics course will be helpful. Since the forward force from the right wheel and the backwards force from the left wheel are not colinear, they create a torque moment which causes the vehicle to turn left as it is strafing left.

As I see it the only reason i would need 4 mecanum wheels is to turn, not to strafe.

If you want to strafe without turning, the rear wheels are necessary. To strafe to the right, the front left and rear right are driven forward, and the front right and rear left are driven backward. The front left and rear right generate forward and right force components; the front right and rear left generate backward and right force components. The backward force component of the front right wheel is colinear with and cancels the forward force component of the right rear wheel; the forward force component of the left front wheel is colinear with and cancels the backward force component of the left rear wheel. What remains is the right force component of each of the four wheels. So the vehicle strafes to the right, with no torque to cause turning.