Yep, I would also say it depends on the deck, as different decks seem to emphasize different aspects of this card. For example, in my beloved Anna.K the meaning is very clear: having too much. In the card, a wealthy man is sitting at a table full of delicious food, and his servants are bringing more to the table. The guy, however, looks very dissatisfied. Maybe he's got too much - or maybe he's got a lot, but not the particular thing he wants the most.
So, if I was using this deck, I'd totally say "can't stop thinking about it". With Rider-Waite, however, it could also mean "isn't on their mind" or "doesn't even realize it", as has already been discussed.
If I was going by Kabbalistic associations, I would also say "can't stop thinking about it". 4 is a number of order and structure, and the element of Water (emotions) doesn't really like being structured. It's in its nature to move and flow freely. When you're totally overthinking something, you're often trying to impose a structure over your emotions and thoughts, you're restricting them by thinking too much - something that won't lead to satisfaction, just further frustration.