Small update, did some testing after installing Windows 10:
Bear with me here, I'll refer to the 4 ports on the GPU as:
1. HDMI
2. DP 1
3. DP 2
4. DP 3
I've done more testing.
If I boot the PC up with one screen plugged into the various ports, I get:
1. Picture
2. No Picture
3. Picture
4. No Picture
If I boot the PC up with 2 screens plugged in (one always in the HDMI port), I get:
1. Picture
2. No Picture
3. No Picture
4. No Picture
If I plug in 1 screen to the working DP port (3) and boot, I get an image.
Then if I plug the other screen into the HDMI while the PC is running, I get a 2nd image.
If I plug in 1 screen to the HDMI port (1) and boot,. I get an image.
Then if I plug the other screen into any of the DP ports while the PC is running, I do not get a 2nd picture.
If I plug in 1 screen to the working DP port (3) and boot, I get an image.
If I then move that cable to the other DP ports, I get no picture at all, even when returning to the working port (3).