Inline code syntax highlighting in GitHub markdown?

GitHub comments, wikis, README.md etc. use GFM, essentially CommonMark with some extensions. There it's not possible. (Follow the link to see whether anything has changed, but don't hold your breath because nothing has in the last decade.)

However, GitHub Pages uses Jekyll and by extension kramdown where you can use:

`x = 4`{:.ruby}

P.S. If you happen to use pandoc, the syntax is:

`x = 4`{.ruby}