What are the differences between the various editions of GURPS?

What are the significant differences between GURPS 4th edition and earlier editions?

The recent spate of GURPS questions have piqued my curiosity, and the system looks like it would suit a campaign I’m planning that would start with a world-generation session using a game of Microscope. GURPS looks like it can handle anything the group could possibly create during the Microscope session.

I’m likely to use GURPS 4th edition since that’s the most recently-published edition, but I’m not shy about using older editions of games if they suit my tastes better. Having no prior experience with GURPS, I’d like to know a bit more about the history of the editions and the changes they’ve introduced before I invest in a particular edition’s books.

I’m hoping that this question will be generally useful for people wondering about the differences between editions, not just useful for my particular situation. I deliberately didn’t phrase this as a system-recommendation question for that reason (though recommendations are not an unwelcome bonus!).