The wording on direct hit has changed in ti4 to be able to target any player instead of only being able to target your opponent.

Is it intended that players not involved in the current combat can use direct hit on ine if the ships involved in combat?

No. Direct Hit explicitly says you play it after "a hit produced by your units." If you aren't in the fight, you don't have any units producing hits in this fight!

Omg. I can’t believe we missed that. Our group has discussed this extensively and somehow we never saw the word “your”. I think wewere so focused on the words that didn’t show up “ opponent” like the wording in ti3 used. Thank you for pointing out our obvious oversight.