No, you cannot. Your 4K monitor is already 4X 1080p display. You will not be able to use your setup as a multi-display. Also, your max Resolution on your 980 GTX is, I believe, 2160p.
What you can do is using the 1080p display, close it, and use your 4k display on two different port... however it is going to be a pain and your icons on your desktop are going to be all over the place each time you are going to switch.
Use one or the other. Honestly I would stick to 4k... as I do right now myself.