Some users want to downgrade iPhone 4 from iOS7 to iOS6, but Apple has closed its signing. Actually iPhone 4 with SHSH backup can be downgraded to any compatible firmware using 3uTools. But what should you do without SHSH backup? This tutorial can guide you to downgrade iPhone 4 to iOS6.1.3 without SHSH blobs.
Why iOS6.1.3? Because 3uTools Pro Flash supports iOS6.1.3 flash without SHSH blobs, and provides the “forced iOS downgrade”.
Please note that after the forced downgrade, you should use “3uTools tethered boot” to start your iPhone after it’s turned off.
Notes before downgrade:
Please backup your phone data before downgrade.
Turn off “Find My iPhone” , or you’ll need to use Apple ID to activate your iPhone after flash.
Download the latest 3uTools.
Part A: iOS downgrade
Part B: Jailbreak and tethered boot
Part C: Install plugin “Insomania pro”
Check original post on 3uTools clearly: How to Downgrade iPhone 4 from iOS7 to iOS6.1.3 without SHSH Blobs