4 wheeled robot steering ideas

I have designed a simple 4 wheel chassis for my robotics project. I was wondering however however how it would turn left and right since there are only two motors, one controlling the front wheels and the other controlling the back two wheels (also is it necessary to have motors on back wheels???). I was thinking about using a servo that pulls on the front two wheels and wherever it turns the wheels turn with it except if the wheels are connected into the axle theyd be too stiff to turn. Is there a way I can overcome this or use another method of steering?

also is it necessary to have motors on back wheels???

BMW seem to think so.

I have to ask - if you designed it, why are you asking now how to steer it?

How about a differential and brake arrangement?

You've designed a configuration that is not steerable. Separate the front-axle onto a subframe that the servo can turn perhaps?

Easiest rc car setup imo is one motor for the left side, one motor for the right side. I had a Tyco Rebound RC car that did this setup. That was a fun car. Pretty fast, to me, at the time.

Allows a zero turn radius, unlike those standard rear wheel drive front wheel steer setups.

INTP: Easiest rc car setup imo is one motor for the left side, one motor for the right side.

Yes, that would be referred to as a "skid steer" configuration. You'd need to drive both wheels on either side with the motor or you won't get adequate steering. The other option would be to have either both the front or rear wheels able to caster (like a motorized wheelchair).