@worshipguitar Yeah no - this isn't stealing IMO. Not at 4; and not with the habit of picking things up and putting them in their pocket. Sounds like your child is just experimenting with what their pockets can hold. I totally would have said to the child (upon seeing the marbles) "Hey (name), where did they come from?" (child tells me) "Oh! I see! We put those away ages ago! Did you forget about them? Could you go pop them in the box please/hand them to me?" and that would have been it.
I'm used to kids popping things in their pockets that they're playing with while they go to the toilet, do an activity with me (but want to still hold onto whatever they have), wash their hands, etc. Unless it's something that they shouldn't be "claiming" (like say if it was the ONLY marble) I leave it be, because as long as it comes back, that's whatever to me. I get the "we accidentally took X home" from a parent and I just go "Oh sweet! Thanks for bringing it back!" and I'd only question the parent or say "Hey did X take (toy) home" if it was something big, or something I knew was ours that they were claiming as theirs (like once I had a parent claim a doll was theirs when I knew it was ours cause I had bought it).
The only other time it is actually stealing, and I would have a very gentle conversation with the parent - after trying to stop it on my own at the setting, would be if they were stealing other children's property. I had a trio of girls who would sneak into other children's bags to take trinkets and take them home and when questioned would say that another member of the trio had given it to them as a gift. It wasn't until I caught it red-handed, mentioned it to the parent, and the child lied to her mum's face in front of me and said "No! She's lying! Sam gave it to me as a present" and I had to very gently say "But it has Tess's name on it, and Tess did a show and tell on it this morning my lovely" for mum to go "I wonder about all these other 'gifts'". (This trio all had older sisters which did cause them to act more...mature, but not in a great way... than most of the other 4/5's in the room).