Medicare Assurance of Rx Transitional Assistance Act of 2004 - Amends title XVIII (Medicare) of the Social Security Act (SSA) to provide for the automatic enrollment of Medicare Savings Program (MSP) beneficiaries under SSA title XIX (Medicaid) in the interim prescription drug discount program and the transitional assistance program, making them eligible for the $600 per year in low-income discount card assistance without requiring a separate enrollment process.