I just realized you will need at least 3 recording channels for what you're asking. 1 channel for your guitar, and 2 channels (L + R) for your music player. The MobilePreUSB only has 2 channels. The M-Audio Fast Track Ultra has 4 recording channels, but it's probably overkill for what you need.
Will probably require a bunch of audio jack adapters to fit your headphones and mp4 player. Meaning you will need to split the L+R channels from your mp4 player to get them into the mono inputs of the mixer.