meaning of '4/5' (American slang?)

I would read it as "4 out of 5", meaning there are 5 of something and this is the 4th (maybe the 4th best, maybe the 4th one chronologically) of them.

An equally valid reading would be "4 or 5" meaning this is either the 4th or 5th of an undetermined number of things (or baby mommas).

/ has multiple meanings. Wikipedia lists about 25. As with any word that has multiple meanings (either with the same pronunciation or different) there is a risk of confusion as to how to read it and interpret it. This was confusing because a fraction is the most common meaning when / is placed between digits. As I am sure that 4 or 5 was the intended meaning, four/five would have been clearer (as the fraction would be written in full as four fifths). But Instagram is an environment where brevity is accepted and so we just have to accept "caveat lector" - the reader beware.