How to downgrade to ios 6.1.2 on iphone 4?

Ok, so I stupidly upgraded my ios on my iphone 4 (gsm) to 6.1.3 and now I can't jailbreak it. Is there any way to downgrade on this model?

Well download the ios 6.1.2 here: http://www.iclarified.com/entry/index.php?enid=750.
using iTunes, restore your device and select the downloaded ipsw file.
now when it's restored, download evasi0n jailbreak: http://www.evasi0n.com
and follow the instructions on how to jailbreak…
after that your device should be jailbroken now… Good luck friend…

You can't downgrade to iOS 6.1.2 now you've moved 6.1.3.

There's a jailbreak for your phone though, it's not untethered, but you can jailbreak;

www.isthereajailbreak.com

Shout at it and call it fat