There is alot of confusion going around at the minute about boot rom version and if you can jailbreak or not. At the minute this is what I have found out….
You canNOT jailbreak or unlock an iPhone 3GS, 4G with a new bootrom
If you have not saved your shsh file from a jailbroken 3.1.3 or 3.1.2 you cannot downgrade from iOS4, have even tried custom firmwares and that does not work
If you have spiritJB do not upgrade to iOS4 as it will cause alot of future problems
If you have the older bootrom then you can upgrade to iOS4 and jailbreak it but you have to be on 3.1.2
In all honesty if you currently have a jailbroken iPhone on 3.1.2 or 3.1.3 I would be patient and wait for a proven method that is future proof if there is such a meaning 
To check iBoot Rom Version on Windows
1. Put your phone into DFU mode (hold power/home till it shuts down and release power when display goes off/continue holding home till you hear a sound on your computer)
2. device manager > universal serial bus controllers > apple mobile device
3. Right click on this and go to properties
4. Click details tab and should be in one of the fields below (change the drop down box till you find iBoot information).
On a Mac:
Put the iPhone in DFU mode then navigate to “About this Mac”. From there go to “More Info…”, and select USB on the list on the left. Under the USB menu you can select “Apple Mobile Device (DFU Mode)” and that will bring up information about the iPhone. About half way down it will have the iBoot information: It will either be [iBoot-359.3] or [iBoot-359.3.2] (the 359.3.2 is the new currently unjailbreakable one). To get it out of DFU mode just hold both the home and power button until it powers back on.
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