Basically you DON'T KNOW which one caused the interrupt.

The only way is to first read the state of RB7:RB4 bits into a variable, then Reset the RB Port Change Interrupt Flag Bit in the INTCON register. You may need to do this at the start of your program, and thereafter each time you reset the RBIF Flag in your Interrupt Service Routine. The next time when the RBIF flag trips, compare the current state of RB7:RB4 against what you had previously read.