It's called sibillance, it can be a problem with anything; headphones, source, amp, the music itself.
Try a little lower than 4kHz. Also check to see that the track isn't clipping.
In my experience with the m50 the sibilance you are hearing is in the recording. I've tried different headphones (HD600) and the m50 with better amps and DACs and the sibilance was still present in the same spots.
Are you listening to MP3s or uncompressed music?
It's in the recording most of the time, but some headphones will exhibited it more than others.
Best headphones in this area are IMO: open HD 650, closed K 271.