Say the key sentence plainly
Form 4868 is the request a US citizen or resident files for an automatic extension of time to file. IRS guidance says directly that an extension of time to file is not an extension of time to pay, so put a payment estimate in the same email.
IRS Tax Tip 2020-38 also notes that paying all or part of the tax electronically and indicating it is for an extension can trigger the extension without a separate Form 4868. Check the current IRS guidance for the year before relying on it.

