The notice is only required for holders of consumer accounts.
First, when there is a single bank involved and you have multiple cut off times at different branches, you must disclose the earliest possible cut off time. If you were to make that cut off time earlier, you would have to notify all customers, not just those whose accounts were based at that branch.
Second, your organization's current practice of homogenizing the Regulation CC disclosures does not necessarily mean you could not separate this bank from the other three by sending the notice of change only to its existing customers and revising the initial disclosure for this bank alone.
However, if one of your goals is to maintain the same disclosure for all four institutions, then I agree with Magic, all consumer customers at the four banks are entitled to notice of the change. You also need to revise your initial disclosure. Those opinions become even less flexible if your customers are routinely made aware that they can make deposits of any of the four banks.