Military Reserve Mobilization Income Security Act of 2004 - Amends the Internal Revenue Code to allow members of the National Guard or the Ready Reserve who have been activated for military duty a refundable income tax credit for the lesser of: (1) the excess of such reservist's daily civilian compensation over the reservist's daily military pay while on active duty or full-time National Guard duty; or (2) $54.80 per day. Includes the amount of such credit in the reservist's gross income.