Tire can be one of the issue causing the car turn over. But the chasis design of Slash and T4 is big different, Slash still have high CG than T4 even u set the both car same "height" from the ground. Also the body of short course is much bigger and heavy than the T4, it also will make the CG higher too. I also suggest u to set the car as low as possible if u play the ground in normal places. Or can u try to remove the body to test?