An IRS Form 4868 is a request to extend the filing date for six months. It's always granted and you're expected to pay your estimated taxes in the meantime.
I always owe, since I don't care to give this government a free loan of my money for a year or more at a time. I also have other documentation which isn't always ready by April of the filing year. So I file later on and pay whatever small amount of interest they charge, which is cheaper than a credit card or bank loan.
I’ve heard of people missing it but never the reasons why and I can’t imagine one since I always file ASAP to figure out if I owe or am owed money.