It depends whether you have the old bootrom or new bootrom 3GS.
If you have old bootrom, then yes, redsn0w can untether jailbreak 4.3.5 on your device.
If you have the new bootrom, then no. And you will not be able to downgrade unless you have SHSH Blobs saved for your phone for the version of IOS you want to downgrade to. Have a look in our jailbreaking section, we have lots of good information in our stickies there.
Hope that helps.