We just encountered error on Paypal live account. We cannot proceed with the validation of payment.
Is this error requires update on Return URL?
Error 404 is referring to page not found issue against your own setting page. Return URL for PayPal Express Checkout is designed and set by your preferred webpage. Please refer to the link below for the details.
https://developer.paypal.com/docs/integration/direct/express-checkout/integration-jsv4/