From my experience it does not matter much if you choose to use the C3 or C4 bearings in your 125cc engine, if you use it for MX. Some manufacturers use the C3 and some C4. But, when the engines are modified for higher rpm’s than they are designed for it seems like the C4’s are the best choice.
Something to be aware of is that the clearances in the C3 and C4 bearings DOES overlap each other. So in the worst scenario, if you get a C4 with the tightest production clearances and a C3 with the roomiest production clearances, the C4 will have a tighter clearance than the C3!
Anyway, as you may figure they are pretty close to each other!
Hope you got my point. I sometimes find it hard to explain technical stuff in a simple way, since my English is not that good.
Good luck!