I’ve seen a theory that makes sense to me.
It’s my understanding that the mask on the H4 poster IS the Hero mask used in H1 and H2. Not only that, I think it’s an old photo from H2 that H4 used.
The theory is that DIck Warlock is wearing the mask. If you increase the exposure and brightness you can see someone’s eyes.
The reason the mask looks different than what we see in H2 is because the back slit is not secured shut like we see in the film. I think Dick was kind of particular about having the mask secured shut because he thought the slit being open and exposing his hair looked silly.
The irony of how badass the H4 Poster looks, and how silly floodwater coveralls and a collar standing comically at full attention looks is not lost on many of us.
If the slit being open looked stupid from the back (which it does) close it up for the shots from behind, leave it open for the rest of the film. Just my opinion.
Found an interesting take: The Masks And Mysteries Of Halloween 4: Part II - Michael Myers
Another one: The Infamous Back Lot Photo
There are many threads on this topic.