When restore (or downgrade) firmware of iPhone 4 using SHSH Server.
iTunes will shows Error 1004 while restoring. and, iPhone will be stucked in Recover Mode.
Actually, the firmware has been restored correctly.
You just need to escape from the restore mode only.
RecBoot_v1.2.zip RecBoot_v1.1.zip , it is made by 'The Core-Dev Team'.
and, Run RecBoo.Exe
Just Select [Exit Recovery Mode], Your phone will work normally.
** This software has been written using .NET 4.0 Framework. If you don't installed, you must install dotNetFx40_Full_setup.exe first.