As I have been reading through all the WHFRPG3 material I have bought, I noticed that in the section on priests it discusses post 4th tier professions usually taken by the clergy (including a warrior priest profession). However, as far as I know, those professions do not exist in any products released to date. Am I wrong? Are they in signs of the faith? Alternatively, has there been any news of a product that will, in the future, give access to post rank 3 material?

You're not wrong, to date no official material has been published that covers Rank 4+ careers (including Priests and Wizards). It's one of the few issues I have with WFRP at the moment!