When using MOTU audio interfaces in FCP4, you will notice that the number of audio channels in the Audio/Video Settings section does not reflect how many ins and outs your MOTU interface actually has. This issue, however, only affects the setup in the Audio/Video Settings window.

Workaround: Go to Sequence > Settings> Audio Outputs Tab and set all channels, then configure the output map accordingly in the sequence settings.

FCP4 will warn you that you have set up the sequence with more channels than the device actually has. This warning can be disabled by going to the Audio/Video Settings > A/V Devices and checking, "Do not show warning when audio outputs are greater than audio device channels."