It's quite hard to downgrade.
Just jailbreak and dual-boot it with IOS 6 using coolbooter.
(Make sure you have at least 5.5gb available.)
You need shsh blobs right? Tons of people don't have them.That is not a downgrade, but rather, a dual boot. You have 2 partitions with 2 operating systems on the flash drive at the same time. Each partition will have half of the original storage, and that may not be what OP wants.
Oh and, downgrading the iPhone 4 is not hard at all. Remember, it has a bootrom exploit. Meaning you can force pretty much everything on it.
You need shsh blobs right? Tons of people don't have them.
Also, there's an advantage to dual-booting: You can use apps that don't support IOS 6.