To be able to use more than 2 screens from a HD 7750 card will require tha Eyefinity Feature to be activated.
You do that by
-using the DP output of the board (easy with this card)
-when connecting screen with DP input you can use “normal” or “passive” cables or adapters
-when connecting screen without DP input you need to use “active” adapters

So it should be like this:
Screen 1 : connect to Screen with “normal” or “passive” cables/adapters
Screen 2 : connect to Screen with “normal” or “passive” cables/adapters
Screen 3 : connect to Screen with “active” adapter with eyefinity support liek CAC-1070 + normal cable
Screen 4 : connect to Screen with “active” adapter with eyefinity support liek CAC-1070 + normal cable

CAC-1070 : http://www.club-3d.com/index.php/products/reader.en/product/displayport-12-to-hdmi-20-uhd-active-adapter.html

It works, thanks!