Congratulations, you have rediscovered the extreme difficulty of using skid steering with four traction wheels and a long wheelbase.
The problem is that you have too much side friction and not enough force to overcome it. One solution is to change your gearing to give you more force, though that will reduce your maximum speed.
The quick fix is to replace your front or rear wheels with omniwheels (mecanum wheels could also work, but they're a lot more expensive and might not fit in the same space). This will let you turn easily, though you'll need some driver practice to get used to the fact that the robot will turn around the end still having the traction wheels. Depending on the design of the rest of your robot, you will probably find one end or the other to be more appropriate for the omniwheels. Some teams have found that putting omniwheels on diagonally opposite corners works okay too, which would put the center of rotation at the center of the robot, but still has the risk of the other wheels skidding/jumping while turning.
It would also be possible to modify the surface of one pair of wheels to make them easier to slide, such as cutting horizontal grooves into the tread or wrapping a bunch of zip ties around it. Omniwheels are easier.