I've been using a Zoom H2N for about a decade. I wanted to upgrade, and despite mixed reviews I decided to take a chance on the Zoom M4, as the price has come down to a very affordable point. I picked it up in the Thomann sale and have no regrets at all.
First a quick note on RF Interference. As soon as I received it I tested for RF Noise Interference, which is widely reported to affect the first release of these recorders. I was actually on the fence about this purchase, due to all the bad reviews out there. If you're reading this, you've probably seen the same YouTube videos as me. Fortunately this problem seems to be completely resolved now, and I found no interference in my unit, even when recording right next to a microwave etc.
Recording quality: I mostly picked the Zoom M4, as it has high quality preamps for external mics. I expected to use those, and not the built in condensers. When I received it I compared the built-in mics it to Behringer B5s – a low cost, high quality mic, that I thought would be a good comparison. I found that the Zoom m4 yields a wider stereo effect, while the recording quality/detail was pretty comparable, maybe even slightly sharper. I guess a pair of SDCs will always give you more flexibility but I find myself just using the built-in mics, because the quality is actually very good – a LOT more detail than the H2N, which I upgraded from. For context, I record solo fingerpicked acoustic, in a room with only soft furnishings to deaden reverberation etc, and the built in Zoom M4 mics are very forgiving. Personally, I currently record just to hear and improve my playing – but in the right hands you could make some very high quality recordings.
The 32 bit float is nice, a novelty rather than a must-have; main benefit is that it makes it quicker to get recording as I don't need to set gain.
Negative points: I do think it's a weird looking thing, and the interface is not that intuitive, fresh out of the box, though you get used to it. If you use it for field recordings you might get handling noise (doesn't affect me as I use a tripod or mic stand.) I sometimes worry about dropping it (maybe I didn't worry enough with the H2N?). Those are minor points. Overall, I'm happy, and it sounds great.
