"Halo 4: I Get a Woman!" - Sgt. Mjr. A.J. Johnson, RIP

[DarkHalo003]

Jen stayed with Bungie as I recall.
I think the explicit avoidance of saying whether Jen would voice Cortana was a sign that Jen won't be involved.

Voice actors tend to not have any particular loyalty to a company though. I seriously doubt she wouldn't voice Cortana.