It's better to file, even if you can't pay it, than not to file at all.
The bottom line is this: The worst mistake you can make is to do nothing. Either file a return or get an extension request in, but do it by the deadline. Sitting back and doing nothing is going to cost you a lot more if you owe taxes.
That just means you've got an extra four months to file your return. It doesn't give you any extra time to make your payments.