However, some other XDCAM product may have more and i guess media composer is flexible enough to allow for 8 or even 12.
I have a PMW-500 and had the same problem in FCP7 in the beginning. i then noticed that my default audio setting in FCP7 when creating a new timeline was 8 audio channels (as you can decide to have 4 mono channels or 2 stereo channels). i change this setting to 4 audio and did not have this 'problem' since then.
Maybe can you change this behaviour in your media composer ?