This theory is not mine. (I think)
So remember in the episode 'Befana' when it was Marinette's birthday? Tikki 'created' her a present *and spat it out LOL* that looked like it was half of the ying-yang symbol. The only other symbol like that in the Miraculous world is in the Miracle Box, representing Ladybug and Chat Noir's Miraculouses. So if Adrien's birthday comes around in season four, maybe Plagg will give him the same present, but with the other half. The rest of the season, Marinette's necklace is tucked under her shirt so no one can see it, and Adrien might do the same. But if they come across each other, maybe the necklaces will do some sort of swirly thing with majic and the necklaces will like, complete each other. They might be able to find out their identities like this if Marinette isn't as oblivious as Adrien. (LOL)
Could this be true? Let me know in the reviews.