How to downgrade from ios 6.1.3 to any other ios on iphone 4?

I want to jailbreak my iphone 4 it is running ios 6.1.3 and as far as i have seen there are no legitimate jailbreaks for this ios. So i want to downgrade it but all the guides involve shsh blobs which i don't know what they are or how to get them. And as far as i know my iphone was purchased while it was running ios 6.1.3 that is how i received it. I have been looking for a way on google but have found nothing helpful any help will be appreciated.

Added (1). Hoping to downgrade to 5.1.1 or 6.0-6.1.2.

You won't be able to downgrade your iOS, Apple have stopped signing them.

The best you can do is a factory reset which *may* remove the jailbreak and keep you on 6.1.3 which a new jailbreak is coming out for.

If not, you need to do a full update either on the handset or in iTunes, you'll get iOS 7 then.

The only place to check for real jailbraeks from All the REAL jailbreakers is http://www.isthereajailbreak.com/