Akius the wizard was furious at the girls. so what if he didn't look like a wizard? then he had a thought. if these girls what to make fun of people like him. they will then be what they aren't.
He used a spell to alter the girls. once asleep it will start. in their dreams, they will see a spirit animal. Their current female form will not matter. they will merge with the spirit animal and take on its gender.
when they wake up they will seem normal at first. then the bodies will become an anthro version of the animal. it will either be fast or slow. reality will alter around them so they will have been seen always like that. they will find themselves giving in to their new instincts. the personalities will depend on what animal they got. Only him and the 4 girls will notice the changes.