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.
Actually, re-reading your question now, it seems that you may not even want to record your sound to the computer. If so, then all you need is a portable mixer. Something like this perhaps: http://www.amazon.com/Nady-MM-141-4-CHANNEL-Mini-Mixer/dp/B0009X9H9I/
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.
Here's another possibility: http://www.amazon.com/gp/product/B00006JPD1/