1. Download 3GS 4.1 to your computer.
2. Download tiny umbrella to your computer.
3. Put your phone in DFU mode, shift +restore (if you have PC) or Option + restore (if you have Mac), then point it to 4.1 fw.
4. Itunes will restore your phone.
5. At the end of the restore process, Itunes will give you a 1015 error. hit okay, close Itunes.
6. Open tiny umbrella, click on your device on upper left hand corner, then hit exit recover on the upper right hand corner. Your phone should boot up to activation screen.
7. Use redsn0w 0.9.6rc16 to install Ipad baseband.
8. Use redsn0w 0.9.6rc16 to install Cydia (reason to do it separately because if you do them at the same time, redsn0w might crash.)
9. Install ultrasn0w from Cydia to unlock your phone.
The reason you can downgrade to 4.1 without SHSH blobs save because Apple still sign this firmware. So, you are okay to downgrade.