Sorry to be a little pessimistic, but this is no unusal following forceps delivery. As mentioned by the above posters it may well be due to residual fibrous tissue in the vaginal wall following instrument trauma.
4 months on, is only a very short time and it may well improve over the ensuing months, but I have to warn you that very little can be done medically or surgically to resolve the problem
Make sure that your husband is well lubricated before intercourse.
Time will be the judge in the long run.