“I would wait for them to contact you, once and if they get their [act] together.”
I disagree. The bank is likely to debit the $4K from the account as thought it were a cash withdrawl when it discovers the mistake. If the poster does not have $4K in the account, that may well cause checks to bounce, trigger overdraft protection (and the fees that go with it) etc., depending on the account arrangements.
“On an annual basis banks rob consumers to the tune of BILLIONS in fees and charges.”
Mike, I realize from your various posts on this site you have a very negative view of lenders generally, and banks in particular. But it is simply not accurate that banks “rob” people of billions of dollars. The fees charged are disclosed in the account agreements and can be avoided by keeping the account current, not exceeding your credit limit, not bouncing checks, etc. There are a few banks and other lenders that engage in practices that are meant to gin up the fees they get, and they deserve criticism for that. But that is not the case with MOST lenders.
“So I don't see any reason for you to voluntarily point out a bank error in your favor to them.”
Aside from the problem I pointed out above about the bank debiting the account for the funds, there is a moral obligation to return the funds. First, because the teller may well be required to account for the error, and thus it would not be the bank that suffers. Second, because if the roles were reversed and the bank shorted you $4K, you’d say they have an obligation to fix that as soon as they knew of it. You wouldn’t say that it was ok for them to say "we’'ll just wait until Mike gets his act together and comes after us for the money before we do anything.” If it wouldn’t be ok for them to do it, it wouldn’t be ok for you to do it either.