Today I added 4 channel I2S output support to the library.
Teensy sends 4 audio channels simultaneously.
To use it, you need 2 audio shields... or 2 of anything else that takes I2S digital audio input. With the audio shields, a small hardware mod is needed on the 2nd shield, to route the 2nd stereo data stream to its input, and to configure for a different I2C address to you can control is separately from the 1st board.
Conversation about the development is on this forum thread. The design tool has been updated with "i2s_quad", so you can easily use it with whatever 4 audio channels you can design. The library has a simple example sketch that plays 2 WAV files to both outputs at the same time.
Maybe I ought to create more examples? What things would you like to have 4 channel audio output do?