It's supposedly a standalone film according to this quote from Scott Stuber:

There’s one stand-alone that we’re working on right now that we’re once again trying to get the script right, but I like it very much, and so does the team. So I feel like if we can get that script right there would be a great kind of extension of that franchise.

Seeing as it's supposedly standalone, i t shouldn't be added right now. If the movie does end up being produced, it should only be added if it's either a numbered sequel or it's marketed as a proper sequel to the first three films.