Wasn't aware of a filter. I saw a Selena shirt from Mexico that looked 4CP but was just blue and red. I did some experimenting and basically you copy the cyan and magenta channels and paste them into a custom blue and red channel, respectively. You still have to get jiggy with the levels/curves to try to get it as close to full color. I did a poster that way; came out pretty sweet.
A sama lama lama lama doo ma see ma nama lama doo ma lama nama see ma
Na ma lama doo ma lama see ma lama see ma doo ma humma, yeah (Yeah)
That's it (That's it)
Here's the one I did:
http://www.gigposters.com/posters.php?poster=22150