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