Is it possible to add a third and fourth PIAA lamp to an existing wiring harness and just use the same switch for all of them? If so let me know how I can set that up.  

I wouldn't.

The harness that comes with the lights is only rated to handle those lights.

What you can do is use part of a second harness and then use one switch to trigger the two relays. That can be done and is perfectly fine.