1.I strongly disagree with viji..at 4 months pups shouldnt be taken for long walks or exercised lots on leash.It should only be free run and play for as long as he can manage happily and not forced walks.Also in case of your puppy it may aggravate the problem and result in permanent damage.
2. Regarding the curve, can't say without proper inspection, but can be either down pasterns ( Where there is excessive bending as if the legs cant bear the weight of the body), just a temporary phase or Hip Dysplasia. Take him to a vet immediately and get him X Rayed.Recovery and medication or supplements will depend on cause as diagnosed by vet.
3.Is your pup over weight, they tend to grow real quickly and the limbs are not able to bear the sudden increase in weight.
Take him to a vet immediately.