The moxf8 could be very interesting, I wonder how it is to play piano on it? I'm thinkin of the cp4 but maybe the moxf8 could be an alternative. If the pianosound and feel is good there's a lot more you can do with the moxf.
Strictly as a piano, the CP4 has the edge with a much better action. The GHS action of the MOXF8 is their low end action, the same kind they put on the P35/P105. I think that, as a portable DP, that might be the biggest distinction, as the MOXF8 is the most expensive one they make with a bottom of the line action, while the CP4 is the least expensive one they have made with a top of the line action.
Comparing sound would be tricker. Out of the box, the CP4 should have the best piano sounds, it has Yamaha's latest piano samples and, I think, assorted piano-specific technology in its sound engine. OTOH, with the flash card, there are some very nice piano samples you can load into the MOXF. Kind of like a Nord, you can load in a wide range of pianos of different characters (though they are generally not free).
Another difference would be in the EPs, where, in addition to the distinction I mad above about the acoustics, the CP4's are more modeled while the MOXF remains strictly sample playback.