"Druids are not absolutely committed to becoming Druidic Priests; they are followers of the religion, and not all of them will aspire to, or achieve, Priesthood" - page 151, Druidic Priests, Warhammer Fantasy Roleplay, 1989
Druidic Priests form the priesthood of the Old Faith.
"There is no major deity or pantheon ruling the Old Faith"
"The Old Faith is based on the natural world, and does not have formal shrines or temples. Instead, there are sacred groves - small clearings in the depths of the forest which serve as places of meeting and worship." and "It is possible for a follower of the Old Faith to gain blessings from sacred groves in the same way as followers of other deities receive them from temples and shrines."
"Characters who are successful in progressing to level 1 Druidic Priest are granted a vision in which a spirit-animal comes to them" - page 206
"Lycanthropy, the condition that turns a normal Human into a Were Creature is frequently caused when a Human somehow becomes possessed by an animal spirit which the Human is unable to control. Druidical priests whose familiar spirit is of the same species as the were-spirit can relieve someone of this curse..." and "If the Druid is successful, the were-spirit is tamed. It is not cast out, but remains with its host in a dormant state." - page 207/8
Druid is a basic career with Career Exits of Druidic Priest, Gamekeeper, Hunter, Outlaw, Trapper, and Woodsman.
So D&D 4th Edition druids could be Druidic Priests who internalize their familiar spirit and sorta voluntarily make themself into a Were Creature.
There is a strong link in Warhammer between druids and the ranger classes. I can't think of any mechanical drawbacks but you could arbitrarily move the Ranger class to be Primal power source especially the Beastmaster version (which could even be a Druidic Priest that focused on its familiar spirit instead of spellcasting). Also druids can ultimately become Targeteers which would fit the Archer Ranger build.
We don't know much about the Warden yet and only dwarf Troll Slayers really feel thematically like 3rd/4th Edition Barbarians. To become a Druidic Priest out of the Druid career requires a roll on the Druidic Advance Table. On that table (page 152) there is a 10% chance the "character is judged unworthy to become or continue as a Druidic Priest, and must move to a randomly-determined Ranger career." So barbarians, wardens, or shamans might be druids who failed to advance as Druidic Priests and instead took up other careers but continue to hold the tenets of the Old Faith.
