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Are you asking for an amplifier that takes an MP4 stream as input or for an audio amplifier that takes decoded MP4 as input? What is it to drive, headphones, stage speakers....? You need to tell us more to get a meaningful answer.
I would suggest an amplifier using two LM386 ICs, one for each channel. They are nothing special as far as performance is concerned but they are very inexpensive and easy to use. They will drive headphones louder than you can bear and also drive small loudspeakers. If you use the application schematic in the data sheet for each channel you wont go far wrong.