Input monitoring of multiple inputs

So I watched the input monitor vid on youtube and it looks like you can monitor inputs 1&2 or 3&4 but not inputs 1,2,3 & 4.

It would be really awesome to plug a stereo guitar rig into 1&2, a keyboard into 3 and a mic into 4 and avoid a mixer altogether. Even if you can only sample one stereo source at a time but could at least hear yourself playing everything all at once.

Cheers