UPDATED - used the correct dark file library this time (time to delete old carp on this computer!!!)
Voila:
https://www.astrobin.com/full/367699/B/
Yeah, it's still only 3 hrs with a 4" refractor, but....
I have also FINALLY tracked down the source of my double diffraction spikes in the right top (3 different scopes but same camera means it HAD to be the camera) - its a very tiny smear of what looks like grease on the window. Will disassemble and clean that off.