I tried doing a factory reset but channel 4 still does nothing. I also had a quick peep at the internals with the fader plate removed, but everything appears to be normal from what I can tell (i.e. doesn’t look like anyone has previously messed with anything).
By experimenting with various settings, I did manage to fix both the ‘slowing down’ behaviour and the double midi messages issue on channel 3, but I can’t remember which settings they were.
I am currently using a workaround since I’m using Traktor in internal mode: I’ve mapped the crossfader (which I don’t use) to the channel 4 volume fader.
I now just need to get the channel levels showing properly in the VU meters - I’ve created a separate topic for that here: Mapping Traktor VU meter levels on DN-X1600