Instead of mailing a check, I suggest you pay online. Go to https://www.irs.gov/payments to make an online payment. Be sure to indicate it is a 2024 Extension Payment.
If you want to mail a 4868, go to: https://www.irs.gov/pub/irs-pdf/f4868.pdf and fill out the form and print it. The instructions with the mailing address are included.