I want to remove the "Pay Monthly" option from the "Pay Later" page and only keep the "Pay in 4" option. Is this possible? I'm using PayPal's JS SDK for integration.
It is not possible. But also, there is absolutely no reason to do it. Such payment options on PayPal checkout pages are between the payer and PayPal. The receiver of the transaction always receives the full amount, and will have no indication whether Pay in 4, Pay Monthly, a credit card, a bank account, or any other funding source was used. All that information is kept private (to the payer and PayPal) by design.