is it too late to investigate a case regarding supposed suicide in jail in nj from 4 years ago?

It may not be too late.   If the other lawyer did not do anything on the case, you may still have time.  The timing starts to run from when you knew, or should have known that there was a problem.  you should seek out a personal injury attorney as soon as possible on behalf of your daughter, who has the claim against her father's estate for the wrongful death.  you will have to become the admininstrator of his estate, and will prosecute the claim as the mother of his child.  Goodluck.