Using 4K iMac as External Monitor (same as Thunderbolt Display)

I have searched the internet for this one, and therefore bring it before you. Can I use a 4K iMac as a external monitor? The caveat here is that I want to use it not as an "additional" monitor, but as the ONLY monitor, the same way I do with my Thunderbolt display. I simply dock my macbook pro into my Henge Dock, and it powers on my Thunderbolt display. Can you do this with the iMac, or ONLY use it as an additional monitor?

Thanks!!