My 5 yr old son has been potty trained for years but has wet the bed 4 times this week. what could it be?

Normal : Occasional bed wetting can still be normal in a five yr old. Sometimes a change in sleep circumstances can lead to wetting, e.g., high level of exercise leading to increased depth of sleep , ingesting a great deal of fluids (as in a hot summer) before bedtime, drinking caffeinated beverages, nightmares. If the wetting persists, check it out with your pediatrician.