I have nothing but trouble since I upgraded from 3.x to 4.x on the 560's and the 565's. Problems from VPN IPsec to very slow web access. Also the SG would reboot trying to make a change. I think FW 3.x was stable and robust. Are there any caveates in downgrading from 4.x to 3.x??
you can ...simply use the -i additional parameter on the upgrade page.
then after the reboot you will need to factory erase, and either restore your latest v3 backup, or reconfigure by hand