I have a similar problem with my cousins iphone 4 which i offered to look at for her. I have been told that the phone was i great working condition until the phone started to install ios 7 at which point the phone has frozen/crashed, and now wont work at all.
The phone does not show any sign of life while not connected via usb (3 different fully charged batteries have been tested). A Couple of time when pluged into the wall socket the phone has booted up to emergency calls only. When plugged into the computer it boots to the apple logo, and then restarts - sometimes it goes to itunes logo.
Now i have tried several different programs that are supposed to help put the iphone into pwned dfu mode, i have also tried normal dfu mode and restore mode, all of which fail to get past the "Preparing iphone for restore" in itunes, as the phone reboots and goes back to loading either the apple logo and restarting or itunes logo. Then itunes displays error 21. At no point have i seen the phone boot to an Apple Logo with a Loading bar. I have tried on several different computers (win xp/ win 7), different usb cables, and the hosts file never has the 74.***.***.** ip's (i have added them to see if it helped then removed them again). Removing the battery for a while then removing also has not worked.
Programs i have tried include:
ireb -windows
Tiny Umbrella
irecovery
sn0wbreeze
redsn0w
SemiRestore
I have been at this for 1/2 weeks now and have searched google and formus high and low and have not found a solution to this specific problem (none that has worked for me anyway)
Thanks in Advance
Steven