iphone 4 is giving error 21 while restoring
 

Hello,

Please can you give any advice on this ?

BR

Google works fine, just ty it !
iTunes: Specific update-and-restore error messages and advanced troubleshooting

I have already tried my friend.
Any other advice ?

Restoring from version XX to XX ???
Provide more details .

The phone had 6.1.3 because i have factory unlocked it.
It seems that during some apps installing ios crashed ( *******r saying ) .
Now i want to restore back, but is giving error 21.

try to change something: PC, cable, battery...

Just make sure you tried software steps and not success. iTunes error 21 is cpu error. You need reball it with stencil.

Update itunes

Change usb try other usb port slot pc

Hang battery in 15 to 30 mins restore again

Cut host...dfu mode. .....Try other pc

he is right, all of them are right, try to change, the problem might be one of the ports or pc :)