Customer brings in $4,000 cash - purchases 2 cashier's checks for 2,000.00 each (same day). Log required?

Under 31 CFR 1010.415 banks are required to verify the identity of persons purchasing monetary instruments for currency in amounts between $3,000 and $10,000, inclusive, and to maintain records of all such sales.

https://bsaaml.ffiec.gov/manual/AssessingComplianceWithBSARegulatoryRequirements/08

When more than one such purchase is made at the same time (or, when the bank has knowledge, on the same business day), they are combined to determine whether the rule applies.

Note also that you have to obtain different info from an individual who is not a deposit customer (you can rely on certain in-file info from a purchaser who is a deposit customer). And the purchaser for purposes of the requirement is the individual conducting the purchase, even if it's being purchased on behalf of another individual or a business.