My friends' dog's hind legs cant move and she's only 4 years old, what can it be?

Call your Vet and ask them if there are any animal help groups in your area.If there are call them all,they will pitch in to help with the cost of a Vet visit and tests.I hope she's OK.Puppy formula can be bought for the pups although they're old enough to start on solid food,mushed up with water...don't give them cows milk !!