Precision 5520, LCD + three external monitors?

I have the same question. I have the Precision 5520 anniversary edition (4k laptop LCD screen with touch interface) and the thunderbolt dock. So, right now I have the integrated laptop screen (4k) and two Dell Precision (std HD) monitors. But I want a third external 4k monitor (total of four). It seems like this is not possible with my current set-up based on another thread I read. I'm only doing std office-type work - no gaming or video production. Is there something like an old fashioned A-B switch I can get to switch between one of my external HD monitors and the other external 4k monitor?

 - Allen