You can only use the primary card while in crossfire mode. There have been special eyefinity cards with 6 outputs in the past (e.g. for the 7850), so if you really need to go 4+, you might want to wait for that.
Source: http://support.amd.com/en-us/search/faq/173 and using Crossfire on 2 7970 with 3 Displays and one TV myself.