* if you want to use the H4n and the videomic to get 3 channels of sound (stereo background from the H4n, dialogue from the videomic), you'll have to use a 3.5mm to XLR adaptor, because if you plug something into the 3.5mm input of the H4n it turns its own mics off,
There aren't too many situations out there that would warrant this kind of recording. If you're recording dialog, you want to record it in mono and as clean as possible (i.e. with as little ambient sound as possible). All the "stereo background" is going to be recreated in post.
If you're recording ambient sound beds, the built-in stereo mics are fine on their own. There will be no need for the addition of a third recording channel.