I also have the 1080 running surround view on three monitors. But my 4th monitor is on a second PC which I use for the virtual cockpit. I've just connected it to the 1080 and it does work. There's a few things you need to do...
Launch P3D and go into windowed mode, you can press Alt-Enter to do that.
Drag the view to cross your three 24" monitors to almost fill them.
Open the GPS window, right click and select undock.
Drag this window to your 4th monitor and stretch it to almost fill it.
Press Alt-Enter again to enter full screen mode again.
If it does not fill all three monitors (It didn't when I tried it)
Go to the Menu - Options - Setting and check the full screen settings are correct....
The Display now has two options, ensure it's on the Surround view one
The Resolution should be the biggest one, I have 5920 x 1080 x 32 for mine.
Press OK and you may have to press Alt enter again.
Mine was a bit reluctant to do it, but it got there in the end.
Hope that helps.