It'll have to be another active dp so you'd need a splitter or hub.
Edit: The xfx page says up to 3 though so that is your max. Do you have igpu to run more monitors?
Edit for spectre's edit: There are 7770 that support 4 (as well as 7750) but it depends on the exact model. Even so, it still has to be on dp for 3+ with any amd card.