Please ignore user "fanfare"'s comment about no need to file a 4868 when a refund is due. While it is true that there is no penalty when a refund is owed, that is really one of those things that is kind of a "relief" when you learn about it and have filed late or are filing at the last minute and miss the deadline by a few minutes.
You absolutely should file an extension if you know that you will be late filing your tax return. Many of your important rights as a taxpayer revolve around "a timely filed tax return", which includes a return with a valid extension. While it may be seldom that this comes up in a meaningful way, when it does, it is a really big deal. So, since you know what an extension is, and how to file it, do so. It's free, it's easy, it protects your rights, and it's actually the law.
