Quite a few websites, Coinbase and Stripe most notably, ask for the last four digits of your social security number to 'verify your identity'. Assuming these websites are not trying to fraud you, how could knowing four digits of your SSN possibly be valuable to them? Both of these websites (if you have made a transaction) have access to your bank account, so does that have anything to do with it?
I think it is here, under 1020.410 (i), which states:
If the originator's bank has knowledge that the person placing the payment order is not the originator, the originator's bank shall obtain and retain a record of the originator's taxpayer identification number (e.g., social security or employer identification number) or, if none, alien identification number or passport number and country of issuance, if known by the person placing the order, or a notation in the record of the lack thereof.
If I interpret this correctly, it means that venmo, coinbase, stripe, etc, need those digits for this in order to legally send money from your bank account.