I'd rather them get to part 4 as soon as possible.
Good things cannot be rushed. You'll get your lowest-stakes Jojo part soon enough. Provided Part 3 is a success enough for them to adapt it. And I don't see it NOT being.
I'd only get worried about Pt7.
Personally Pt4 is my least favorite because it just doesn't have the same weight as the others and doesn't even tie in with the rest of the storyline in any way, shape or form unlike every other part until 7; but each to their own. IMO it feels out of place; and almost like a breather compared to the other sections. I'd explain how the other parts tie in but hey; that would be major spoilers. Even Part 5 which has no direct impact on anything after it is more... cleaning up after what happened before.
I mean Part 4 has a certain item that is important in Part 3 and Part 5 as well; but it's use is not anything that really changes the overall storyline of the entire thing.
You could skip Part 4 and the story would still work entirely. You could do the same about Part 5 too; but Part 5 dosen't make sense without the preceding parts [Except 4]; while Part 4 does.
It's not BAD; it's just my least favorite of a really; really high standard bunch.