Which one is the 4G sim slot in Redmi 3s prime?
The Redmi 3S Prime uses a hybrid SIM slot which means it accepts one Micro-SIM and another Nano-SIM or microSD card for storage. 

Unfortunately, this means that users will have to choose between a second SIM and a microSD card.

Coming to your question : Use any slot for 4G. It is recommended from my side, use the micro-sim slot for the primary sim.