If you're connecting to DVI or VGA monitors, you need to use an ACTIVE Display Port adapter if you wish to get more than 2 monitors working simultaneously.
I had the same issue. This morning 2 x Active MDP to DVI adaptors solved the problem for me.
Although it was still a bit of a challenge... Even using the Adaptors windows 10 native didn't really allow me to make the adjustments needed but the catalyst control centre permitted me to make the changes. Using version 15 drivers on 64 bit downloaded from AMD.
The Microsoft recommended driver (from windows update) that it attempted to download rendered all my USB ports unsable and WIndows 10 safe mode (with an SSD) is not easy to get to with a computer you can't control. Had to use remote desktop access in the end to remove that driver.... geez.
So to sum up - 4 monitors working perfectly with Windows 7 and Windows 8 using pasive adaptors but after Windows 10 upgrade I could only get 2 monitors working. I now have 2 passive and 2 acitve adaptors and using CCC I can get the monitors working again.
... I'm yet to reboot - so I hope the settings stick.
best of luck