My son was also completely potty trained even at night, and then our second child was born. Yep, he backslid at age 4, although mostly only when he was sleeping. So we have gone back to pull-ups for bed. It was definately stress related, he stopped wetting at naps after a few months. And now a year later, he is mostly dry most nights, but every so often, not.
Then he started having to pee all the time, like 4 times in an hour, so back to the doctor. The long and the short of it was nothing wrong, but worth checking.
I also know people with a son who got a strange kidney issue, that with treatment also resolved.
Definately check with pediatrician. Do make the doctor do a UA to rule out bladder infection, UTI, diabeties etc. My son loved peeing in the cup!!
Do not shame him, or punish him, when my son has growth spurts it's worse. At his age, he is not doing on purpose. I would talk to him, and put him in pull-ups if he is comfortable with that. And see if it stress.
The other possibility is that he is getting a long enough attention span, that he doesn't want to stop playing so he waits to long. If you think this is the case, you may just have to start a timer,reminder routine (kindly) to help him through this phase.