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Plugging in an external mic, however, overrides the X-Y pair, which means you can record with the internal MS mics (in whatever stereo or mono configuration you want) AND an external mic or stereo line input at the same time.

If you select 4-channel surround mode AND plug in an external mic (or any other 1/8″ input) the H2N will record two files, each as stereo pairs–one through the MS mics and one through the external input. (Note: you can only use the MS mics at the same time as the external input, as the X-Y mics are disabled by plugging in the external input.)

There are a couple caveats, though:
1) There’s only one gain knob for the entire device, so you can’t adjust the levels for the external input and the built-in MS mic independently. (So obviously you have to set your gain for whichever signal is hotter.)
2) For reasons that aren’t clear to me, you can’t record the MS mics in “raw” mid-side mode. This means that the stereo pair recorded by the internal mics aren’t encoded as MS coming directly out of the H2N. (Though, as Fran mentioned below, you could re-encode them in post.) You can still adjust the stereo width of the MS mics on the device itself before actually hitting “record,” and you can select anywhere from mono cardioid to 150 degree stereo. I suspect the folks at Zoom simply didn’t intend for people to be using the 4-channel “surround” mode for anything other than surround recording with the internal mics. Recording via the external input and the MS mic feels sort of like a serendipitous hack. (And the manual doesn’t seem to mention it.) I’m glad to have it, though, and have already found it useful for recording a concert direct from the mixing board and via the internal mics. It would be nice if Zoom enabled RAW mode in 4-channel mode in a future firmware update.