Yes, print the 4868, complete it, and mail it in. It is the date that you mail it that matters. You might get a "certificate of mailing" to show that you mailed something to the IRS.
You can find the 4868 at https://www.irs.gov/forms-pubs/extension-of-time-to-file-your-tax-return (look for 4868). As for the IRS e-file of the extension, I think you need your 2017 AGI for that, too.
