I had a similar problem after updating to 4.0, you get a 404 Error on this application-… files.
normally the files should be located at /opt/zammad/public/assets/
if the files aren’t there, you can recompile them with zammad run rake assets:precompile
after this the .css file was loading again, but it kept stuck at “loading…”
then I restored one file from backup, and it’s working again, but without the 4.0 features