It would be interesting, so long as it isn't crap.
She's had a pretty good run of blockbusters with the Pirates franchise, so she's bankable. I don't know though, she seems more intent right now on doing films that are more critcally appreciated. I'd hate to see her career derailed by a high profile disaster. Silk was a small film that very few people heard of, so its box office and critical failures passed without notice. JP4 would be a summer blockbuster, and if her part simply features her looking good and pouting (which is a big possibility, most summer blockbusters don't have a lot of script space for dialogue) the same questions will be raised again. And, if it's a box office disaster, it'll compound the negative attention.
I just don't know how much character development there would be in the fourth film of this series.
"Purgatory's kind of like the in-betweeny one. You weren't really shit, but you weren't all that great either. Like Tottenham."
I'll try being nicer...if you'll try being smarter.