Check this out below name HOME CYDIA it writes that I have
shsh blobs 3.1.2 and 3.1.3 so even if I update to version 4 I can still downgrade to 3.1.2 or 3.1.3. I'm sorry I can't upload my screenshot because only iPhone I use for replying here now.
Man were talking about now 3.1.3 coz it's already downgraded from 4.0 so he can already access in cydia because 3.1.3 can be jailbroken 4.0 stil cannot be jailbroken as of now so you can't install cydia. But if you have mac os search forum for instruction of jailbreak 3gs 0s 4.
iReb is from the website ih8sn0w.com, on there is also the useful iForecast and sn0wbreeze.
What the threadstarter has failed to explain is how he got past the error msg 'iphone the device is not eligible for the requested build' when trying to downgrade the 3Gs.
Everything is basically correct in theory.