1) Old/new boot: this is just to find out if it's tethered or not. To find out old/new bootrom you need to put your phone manually to DFU mode (not restore mode) and read some values in the USB strings.
Detailed instructions:
which-bootrom-do-i-have-3gs | jailbreakmatrix.com
2) To jailbreak iOS4 you need to downgrade to 3.1.2 first.
3) To downgrade 3GS firmware, you need the Apple certificate for your iPhone for the firmware you want to downgrade to (shsh file). Apple created certificates for 3.1.2 until Feb 2, 2010. If you didn't back it up until then, you cannot downgrade. If you were jailbroken before that date, Cydia did automatically back up the shsh file. To downgrade using shsh, the post above is correct.
I hope you backup your 4.0.1 shsh now to downgrade in the future to that version, if you forgot it earlier.
See timeline for shsh:
SHSH - The iPhone Wiki
(the rest there is technical)