In my area,in the early Spring, the top 1 or two inches of soil will thaw during the day but the ground underneath will remain frozen and let me tell you does it ever get greasy.I've observed in those conditions that a 4T will just sit and spin ,even on level ground,whereas a 2T will somehow find traction and pull away. It's actually a dangerous time to ride and I once got speared in the calf by a pine branch that found it's way past my safety gear.