Did you get Archives of the Empire, Volume III? If you didn't it has nice articles about the The Old Faith (and folk workship & Hedgecraft). Article also has rules about Priests of the Old Faith, but what I was missing was opportunity to present Druidic Priest of the Old Faith -career. But sadly no... Well, no worries. This is my take on that career. It uses the rules given in the book, but also it has good old Spirit/Totem Animal Familiar rules from the WFRP1 turned into WFRP4 rules. It should make this career interesting option. And I have made few new rules detailing about the Spirit Familiar.
WFRP4 Career: Druidic Priest of the Old Faith, Version 1.0 (PDF).