I filed for extension through TurboTax (Form 4868) and see box 8 "out of the country and a U.S. citizen or resident" unchecked when it should be checked. Can I amend?

Related question: Form 4868 provides a 4 month extension for those "out of the country." I am out of the country so my understanding is that my extension will be April 15 + 4 months, or August 15, 2019. However, I plan to file Form 2350, which will allow me to file in October 2019, giving me enough time to qualify for the foreign income exclusion act as I moved overseas in October 2018. That being said, should I have not submitted Form 4868 which only gives me a 4 month extension?

I have a $500 estimated tax payment submitted through Form 4868, so would like to understand if I should cancel Form 4868 and only submit Form 2350.