It very well is probably just age regression. Which I chaulk up to them realizing they are growing up and kind of slipping backwards a tad to be a baby again so to speak.
You can play into it and just let him know that is fine, if he isn't going to do big boy things then he isn't a true big boy and remove big boy toys, take away big boy priviledges.
Be matter of fact, don't get too stressed and stay calm. Don't punish for accidents, however let him know that you are disappointed as you thought he was potty trained! Put him back into pullups!! That will save your sanity and until he is ready to do the work, you aren't having to clean up messes. Some kids take longer too to get all of the urges down. Pullups made true potty training so much less stressful for myself and my kids. When they were ready, it was done and there was no looking back, it was that easy. I just followed their cues however. Both were 100% trained day and night by 3 1/2!
Just look at him and tell him you KNOW he can do these things, if he is choosing not to then that is fine but you don't allow big boy things without truly acting like one.
I am willing to bet a week without anything but baby toys and having big boy priviledges taken away, well he may change his mind.
Also, if you celebrate Christmas, have an outside source, (friend or relative) call him, pretending to be Santa, tell him that they would be really proud of him if he was a big boy, wore big boy underwear and acted like a big boy so he could bring big boy toys. Sounds odd, but it turned my daughter around 180 in a matter of one hour! No joke! She regressed due to wanting my time after her little brother was born and I was okay with it, but had a local rec center who lines up senior cititzens to call your house as Santa to call my daughter, told her how he wanted her in big girl panties so he could bring the cool big girl toys and when she hung up it was over with!! Not one accident after that.
If you are concerned, if he seems really withdrawn or seems like he is truly having issues, take him in to the Pediatrician just in case. Since he is almost 4 you can make it his 4 year checkup but talk to the Peditrician about what is happening.
Hang in there. They I think want to be big kids then realize it is kind of scary and regress in all areas sometimes.
Reassurance, lot's of love and cuddles and reminding him how proud you are of him when he does for himself then that may help!! :)