A South Jersey man was sentenced to state prison on Friday in the sexual assault of a four-year-old girl. Marvin Stidham, 64, of Commercial Township, has been sentenced to 17 years on two separate sentences that will run concurrently, the Cumberland County Prosecutor’s Office said in a news release on Monday. A jury convicted him of second-degree sexual assault and endangering the welfare of a child in January. He was acquitted of first-degree sexual assault. An investigation by State Police and the county prosecutor’s office determined that Stidham had sexual contact with the child in Commerc…