Font Awesome (v4.7.x) does not have the Gitter icon. They introduced it from v5. Now, without entirely updating all of the fontawesome files or using the v5 cdn, how can I just use the fa-gitter icon?
I do not want so say it's not possible - but I'd say it's beyond most FA-Users (including myself) to embark on such an adventure. I don't think one could get away with "just one icon" - there's always CSS involved...and the CSS from FA4 and FA5 (may) overlap and could cause further trouble - this really feels like an ugly can of worms.
Your time is probably better spent researching and eliminating reasons for not upgrading than trying to mix the two.