My 5 yr old son has been potty trained for years but has wet the bed 4 times this week. What is going on?

Probably normal. Many children, up yo age 5 or 6 years "relepse" and have a few wet nights. Your son may have started to drink a lot beforegoing to bed. May be excessively tired and thus not have the sensation of a full bladder. Certainly have his urine checked in case of infection, however his urine would likely have a strong odor if infected. Utis in 5-y-o boys is quite rare.
Consider many things. What else is going on in your son's life this week? Is his schedule more hectic? Children may wet the bed when they are overly tired or drink too much late at night. Is he upset about something? Are there any signs of illness? Urinary tract infections or even diabetes could present with bedwetting. Don't hesitate to discuss this with your doctor, if it persists.