Can I still downgrade from iOS 6.1.3 to iOS 6.1.2 (iPhone 4)?
As mentioned in the firmware signing status for iPhone 4, only iOS versions from 7 and up are being signed by Apple.

In order to use redsn0w to do a downgrade, you need the SHSH blobs saved for the specific version that you wish to downgrade to. In this case, it seems like they were never saved while the phone was on iOS 6.1.2. So you cannot create/retrieve them.