Not sure if this as issue that affects any VMS4 models, but with my VMS2, the headphone volume is extremely high. High to the point that you worry if the knob got nudged or bumped at all, you could really hurt your ears.
Of course, this is affected by the loudness of what you have running through it, but I actually have my MP3s gained to a pretty low volume of 92.0 dB, or +3, and this is lower than a lot of apps like Spotify or SoundCloud output.
At 1/4 it's got okay output and anywhere even approaching 1/2 of the way up and it's loud enough for DJing in a loud room. Anything above 1/2 and it'd be dangerously loud, so I imagine a more natural and hearing-friendly curve would be nice, with only 100% being "very loud."