I wish I could tell you for sure, but I'm afraid I'm newer than most to the game and the TLG site... the only version other than the most recent (4th) I own is the 3rd Printing, so I couldn't tell you all the way back to 1st, but from what I gather, the core of the game (the Siege engine-- Prime...
4th printing. Big difference.
There are some very minor differences between printings; usually editing cleanups, minor rules nudges and the like. But the rules themselves have gone through very few of the grand, sweeping changes or overhauls that one would expect from an edition change.