I downgraded successfully with iTunes 10.5.
Download 4.3.5 ipsw and tinyumbrella (uncheck mark "set hosts to cydia on exit" in advanced tab. Put phone into DFU mode, open itunes + hold shift key on restore button, choose 4.3.5 ipsw and you're good!
Download 4.3.5 ipsw and tinyumbrella (uncheck mark "set hosts to cydia on exit" in advanced tab. Put phone into DFU mode, open itunes + hold shift key on restore button, choose 4.3.5 ipsw and you're good!