From the front of both units, other than in name there appears to be little difference between each model, however on the rear panel the difference becomes apparent .
The UMC202HD has minimal options: such as USB, phantom power and 2 outputs....
....whereas the UMC204HD has USB, phantom power and offers 4 outputs in MIDI IN/OUT, playback outputs and main out plus the added option of inserts which allow the user to send the signal to an external outboard FX unit (such as a reverb or EQ).
A full product description for the UMC202HD is available here.
A full product description for the UMC204HD is available here.
Keywords: UMC202HD, UMC204HD