Usually there is swelling with slipped tendon. Can she put weight on her leg? what all have you done to help? have you been soaking and or using Epsom salt wraps? massage?
https://www.backyardchickens.com/threads/successful-treatment-of-slipped-tendons-in-bantam-duckling.987977/
Is there swelling in the knee area?
What I'd like to see is a video.
But it doesn’t feel dislocated? At the hip? Use the Epsom salt compresses on her today and see if they help. Try not to let her run or try to run around Rest is important. If you notice redness, heat and more swelling then I’d try getting her to a vet. Leg injuries can take time to heal so...
Make sure any water you use is nice and warm, putting a 4 day old duckling in water can make them chill fast so always keep close tabs on the warmth of the water so you get the duckling out before the water loses its warmth.
Rest and warm water therapy. Also, some liquid B complex is so good...