I have been using my Zoom 4Hn for about 1yr, same SD card.
I just did a job last week and used it a couple of times with a Lav mic. The first recording worked fine and the second one seemed to be fine.
After recording I played it back, as i couldn't monitor it live, the first 20secs (of about 4 mins) were ok on play back. later that day I went to download the .wav file from my card and it wasn't there.
I searched everywhere within the SD card, but to no avail. I have since used file rescue software, it found a lot of files but not the one I needed.