We have a 4 hr lay over in Naples via train and like to check out the city. Is it safe to leave your luggage at the luggage storage in the train station?
It's a manned deposit, not lockers.
In my experience yes. As Dario said it's a manned deposit where you give your luggage to someone, they give you a ticket and store it in the back. It was very easy and professional.
We had a 4 hour layover there also, and we used the Luggage storage place there, no lockers just attendants who take your luggage and give you a ticket to retrieve. It was safe and reliable, but I remember it being a little pricey. We used our 4 hours to get Pizza and get back to the train station with time to get our next train.