I tried to upgrade my 5.0.1 iPhone4 using Snow, and was dismayed to see that 6.1.2 is no longer accepted by Apple for singing to restore. 6.1.3 is not untethered for an iPhone4. Any ideas?
That's always the case.
You cannot restore to a firmware version that is not signed by Apple.
For the iphone 4 and 3GS if you saved your shsh blobs while 6.1.2 was still signed you could then restore to that version even if Apple closed the window.
Not much you can do if you don't have the 6.1.2 blobs for that specific device. Only option would be to stay where you are or go up to 6.1.3 and perform a tethered JB.
Thank you very much for the quick response -- you save me a great deal of time searching and potentially breaking something -- greatly appreciated. I will start to monitor the jail-break scene on a fairly regular basis so that I don't miss out on the next opportunity