How to Update iPhone 4 to iOS 6 (Not iOS 8)

Question

I have an older iPhone 4 that needs an update.
it's running iOS 5.1 and i'd like to update it to iOS 6.3 (NOT iOS 8)
I downloaded the iOS 6.3 .ipsw
I held the Option button down and pressed Update in iTunes looked for the file and tring installing that, but after a minute i get an error message. Not eligible.
I'd expect iOS to not be Eligible cause the hardware can't keep up. but iOS 6? wasn't that the best build for the 4?
does this mean the only way to update my phone now is to Jail break it?
BTW, i don't use this as a phone anymore, I use it as an iPod.
Any help is appreciated.

Answer 1

being that it's an iPhone 4 I don't think Apple can (or would) help me with anything at this point anyway. they don't have a 4 year warranty. And i wouldn't expect Apple to help at this point anyway.
I would however expect Apple to let it's customers do what they want with their products outside of the warranty rather than lock them down.
Regardless. It looks like I'll be going to 7.1.2. anyway
I want to use this as an iPod, (90% for Music) so SPEED is not as much of an issue anymore.
Spotify requires iOS 7 so i guess I'll give in.
hope this doesn't bust the phone like it's done to so many others out there.
For reference, here is a short list SPEED difference from iOS 6.1 to 7.1
http://www.cnet.com/news/want-your-iphone-4-to-run-faster-install-ios-7-1/