I owned first and third editions, and played a fair bit of first edition. I'm sort of lightly interested in fourth edition, but am curious about what's been changed, fixed, or altered. If the question's too large, I'd settle for the differences between third and fourth and will edit the question accordingly.
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4th edition was redesigned as a toolbox rather than focus on a single time period.
Mechanical changes between 3rd and 4th are mostly there to present a more clarified rules structure. Changes that will become apparent during play: void now is the cap for raises(higher of void or skill rank in 3rd), void price is x6(x4 in 3rd), free raises are very rare now, increased damage maneuver adds only rolled dice, all the bushi schools are more in sync as their abilities seem to grow at a similar rate between each other, minor changes to carapace, honor glory and shadowlands are all based on 10 ranks instead of five, some attribute bonuses from families and schools have been changed, actions are now divided into free simple and complex with chart to define what each does.
4th edition is a great system to introduce new players into Rokugan. It's structure allows for more attention to be placed on the actual story, and the toolbox design allows you to visit any time period in Rokugan history(or alternative history) with only minor changes.
For players that have played 3rd or 3rd rev. and enjoy that system there will likely be a feeling of the nerf hammer dropping on your character. Certain combos, weapon styles will need to be rethought if you make the change.