Downgrade from 4.0 beta 2 to 3.1.3

[nil911]

hey guys,
i know there might be many people looking for this.
i really want to go back to 3.1.3 version, because i am having network issue and apple can only fix it if im running 3.1.3 firmware. hope to get helped from some1 soon, i tried going directly with itunes but i get an error. hope some1 can help.
or atleast tell me if its even possible

[whereswaldo]

sometimes even itunes makes mistakes, try again

Edit:
I searched around and found this
For Windows:
iPhone Jailbreak Guides | iPhone Tutorials | ModMyi.com Blog Archive Restore iPhone 3GS to 3.1.2 Using SHSHs on File with Cydia | Windows
For OS X:
iPhone Jailbreak Guides | iPhone Tutorials | ModMyi.com Blog Archive Restore iPhone 3GS to 3.1.2 Using SHSHs on File with Cydia | OS X
this doesn't just work on the 3GS, i have tried it on other devices and it isn't only for restoring to 3.1.2 it works on everything
P.S. if u don't no what an SHSH is, you can just ask, i didn't know until recently

[nil911]

thanks i will try when i get hold of mac, since windows is not supported by new firmware ... n there were always issues with people with windows so i have to wait to get mac

and no i donot know what SHSH is i heard its something that you might have had it from cydia or something

[whereswaldo]

A SHSH is I file deep inside your iPhone recording what firmware you are on, cydia records this and then keeps it for that device, so as long as this iDevice was once on 3.1.2 and has been jailbroken since then cydia will hav it on file and you should be able to do this
note: I cannot vouch for this working on a mac because I have a PC but it has worked for hundreds of other people so it should work for u

[geiman]

I just did this last night on my 3GS; I ended up having to downgrade to 3.1.2, then restore to 3.1.3 and jailbreak. I used Pwnage to do the initial jailbreaking (restoring to custom firmwares) and then ran redsn0w to get verbose boot and tethering on 3.1.3.

All is working happily again...