I have never built a rover so pardon the ignorance.
First. Is it possible to build a 4 wheel rover that uses differential steering ie like a tank. Not sure if differential steering is the correct word. I need it to do a pin point turn in a small area preferably on the spot.
Second. I have a couple of brushless motors lying around. Can that be used to drive the 4 wheel of the rover as above?
The YouTube channel rctestflight has several ground rover projects. You might want to watch some of his videos. You can see some of the problems with using four wheels in the linked video. He had to shorten the robot in order to get the robot to turn well. He has several other ground robots powered by brushless motors. Most brushless motors will require a gearbox to drive a ground based robot.
The tank like steering is called “skid steering” and it is supported (see here). Brushless motors meant for flying drones will normally not have enough torque to work well. I’m pretty sure that modern electric cars use brushless motors though so it really depends upon the motor.
There are numerous six-wheel rovers out there doing skid-steer turns.
Look at power-wheel chair based builds. The center two wheels are the drive wheels with independent drive (24v brushed DC motors and gear boxes typically) with a pair of full swivel wheels in front and back (originally for stability when hauling human cargo)
One thing I would add here, even though no one has asked, is that it’s probably useful to take into account the sort of terrain you’ll be running on and the weight of the machine you’re building when thinking about skid-steer.
In my case, I built a mower. I started with a skid-steer configuration but found that with the weight that was on it (which was necessary for traction given the mower deck it was pulling around), it had a very difficult time turning on grass. (Which makes sense when you think about it. How many field tractors have you seen that turn in a field by skidding?) Additionally, the effort required to turn any vehicle beyond very small size/weight using skidding has a noticeable impact on battery life.
I ended up building a 2WD vehicle with a caster on the “steering” end, using differential torque to steer. Configuration for ArduPilot is similar to skid-steer configuration and is well supported. The caster offers no resistance in turns, and skidding is not required. Total vehicle weight (without the mower deck) is ~70 lbs, and it turns with very little effort in grass (and also doesn’t tear grass up from all of the skidding and wheelspin). Battery life (2 wheelchair motors for propulsion, 18Ah li-ion pack, 60A motor driver) is ~3.5 hours or more with no mower deck attached. With the mower deck in place, runtime drops to ~1.5 hours in my yard (which is hilly). Current draw on level ground is ~2A, ~6A on hills, and roughly ~20A during turns as it swings the 2-meter long mower deck frame around.
Anyway, just thought I would throw that out there.