What happens to the speed when the two small people are placed on one side and the two taller people are placed on the other side? what about when it is one small person on one side versus all four people on the other side?

People of lighter weight are sitting on one side and taller people are sitting on the other side. There will be a difference in the balance of weight in the vehicle, if this continues to remain the same.

Then the speed increases the short people having lesser weight would feel a lesser gravitational pull. while the other one with heavier weight will feel more gravitational pull due to which the vehicle can turn over due to imbalance.  

the answers would be

the first one and the second one