What do you mean by hash? 4chan posts use an incrementing id per board, so if you put >>>/sci/3751197 you have a unique identifier for the post (sci being the board and 3751197 being the post number). For convenience you could link to an archive site as well, but they're known to die every few years.

Some boards like /pol/ have a user hash, unique per user-session, stable across posts.

The hashes assigned to your IP on the boards that have them are per-thread to my knowledge. All your posts in one thread will have a particular hash, but your post in another thread will have a different one.

Ah I forgot about that. It would certainly be a good idea to include it if available. Do you know if it is necessary for a unique id? I've never browsed any of the boards with user hashes.

Not sure what you mean by necessary. Post id would still be unique.

You'd need a link for people to see the original content too so you'd want to include a link to one of the archive sites I guess (as a second best source for an ephemeral post). It's odd but there are citation patterns for websites, wonder if they include a standard for including archives of sources not available at time of writing.