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 for their legs and has the niacin they need to keep those muscles strong as they are growing. TSC carries one it will say injectable but we use it orally. You can put it over their feed or in their water but with one having an injury I'd put it where you know she is getting full amount she needs.
Scrambled eggs are so nutritious for ducklings also along with reg feed. Can you post a picture of her and the leg in question? Also instead of putting Epsom salt in her bathwater use and washcloth and soak it in some warm ES water and wrap the leg a few times a day. That way you don't have to worry about her drinking the water. Just leave the wrap on till the washcloth starts cooling down. wrap your little one in a soft small towel while doing this.
Any time you are holding a duckling place your hands around the whole body like a wrap so the little one can't jump or squirm out of your hands and take a fall. They are fast and as you have found out it can happen super fast.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
this is info on niacin deficiency but the reason ai am adding it is because it tells you about the B complex and how much to give your duckling at a specific age.
This is important info anyway because many don't know that if we have feed made for chickens our waterfowl babies aren't getting enough niacin and can have leg issues from it.
