I had a bit of trouble figuring out why my preprocess-html.inc file wasn't working in my first Omega 4 subtheme. I found this blogpost and fubhy's comment pointed me to the base theme for examples, there I learnt the file naming was different.
I noticed the readme's in Omega 4.x just say 'Fill me.' so I've modified the Omega 3.x /process & /preprocess readme files to indicate the new naming conventions, hope this helps! Thanks for the great theme!
Omega 3.x preprocess.inc file naming:
Omega 4.x preprocess.inc file naming: