Let's call first set A and B and new set C and D. You'd better place them in the same room.
1. To add C in A and B, press pair button on C for 1s and press pair button on B or A for 1s. Powerline light should be flashing for a while. Wait for about 60 seconds while your Powerline adapters are connecting. The Power LED on Powerline adapter A/B and C will stop flashing and become solid light when the connection is made.
2. To add D, you can repeat above steps.