I've had Parallels in coherence mode with a Windows 7 guest work fine for me with 4 screens. However, I did run into a problem similar to the one you described where it wouldn't start. I found out that it was related to what screen the guest OS window was on when I tried to enter coherence mode.
When I had the window on the main Mac screen (the one with the menu bar), I could enter coherence mode just fine. However, when I moved the window onto one of my other screens, it would fail when I tried to switch.
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