All the monitors must be the same resolution for Eyefinity, but there's no problem configuring the 4th monitor to be non Eyefinity.
One monitor must support the Displayport, or you'll need an Active DP to HDMI/DVI adapter to run Eyefinity.
No, HD 6870s in CF is very weak for such a resolution, you might consider Crossfiring 2 high end cards such as HD 7950s or GTX 670s.
Also, read this about Eyefinity. http://www.tomshardware.com/forum/344343-33-eyefinity-setup-guide