I believe it is on page 71 of the manual: Input / monitor / On. No extra equipment needed. ( Disclaimer: I have not used the H4n ).
It sounds like what you need is a noise gate. There are several (even free) VST's out there that could achieve the result you are looking for. You could either record your audio into any DAW running such a VST, or apply it to previously recorded audio. Here's an article with more explanation and some recommendations for VST's: https://integraudio.com/10-best-...
Purchase a splitter adapter that takes your mic output and splits it into left and right channel. If your mic is mono, it will just send the same signal down both.
Like Winston Chow mentioned, you could use soundflower to re-route your audio but if that's too much hassle, the hardware solution may be a much better, simpler solution.