Q: Can the 4k iMac be used as an external display for a PC laptop?

That function, Target Display Mode, is a Mac-to-Mac function and wont work with a non-Apple computer:

Use your iMac as a display with Target Display Mode - Apple Support

aaronminnick wrote:

 

So just to be clear, there wouldn't be any way to use a thunderbolt to HDMI/Mini-display port to mirror my non-Apple computer onto the new iMac?

Correct, read the link that was provided earlier. Apple makes this perfectly clear.

no but you can do it using programs such as avatron air display I use that to make my ipad2 act as a monitor

Another alternative to use the iMac as an interface to a Windows machine is to use Microsoft Remote Desktop Mac.

Um, Guys...

The 4k iMac cannot be used in Target Display Mode _at all_ anyways... So the PC question is moot.

Apple removed TDM from 4k and 5k iMacs!

Please correct me if I'm wrong, because I was disappointed to say the least.

Always loved having TDM around for MacMini and old MacPro.

Use MRD instead

Forget that, it's the wrong way around, use Teamviewer (use the lan connection option)