How to sense RPM from 4 pin BLDC motor?
Your '4 pin BLDC motor' is probably just a normal 3 phase motor wired in Star or 'Y' configuration with the center tap brought out. If your motor has phases that are 4&ohm; each then it should measure 4&ohm; from center tap to each phase, but 8&ohm; between phases. 

Some drivers use the center tap to help measure back-emf for commutation. However a 'virtual center tap' can easily be generated externally, so most modern drivers only need 3 wires. The TDA5144 is an example of a driver IC that is designed to work with 4 wire 3 phase motors.