Hey philipma1957,
Thanks for the reply! Yep, they are 1060s. Another one might fit, it will be tight.
PSU is indeed the one you mentioned.
The 5th card doesnt have to be the same model, right? As long as its not AMD I guess..
The fifth card does not have to be the same model, or even use the same GPU. You could add an AMD GPU, and it's relatively easy to put together on Windows, but it can be difficult to juggle the drivers on most Linux distros.
The one you can choose will largely be restricted by available power. I think you could get by with another 1060 (count your connectors, too).
Finding a good spot to mount another card is up to you, though.