Hi Rune...if you can get a 3rd ed., grab it! According to the fly leaf on the 4th ed. the changes are: freshened illustrations, repairs for watches which are peculiar and for which there has been no previous instruction, negative setting , motor barrels, dismantling and assembly of a verge fusee watch. It very well be that some of these topics now have expanded coverage.
I've heard some people prefer the earliest editions when they are into repairing earlier watches (pocket watches and early wrist) because there is more in depth on this type of repair rather than some of the swiss wristies covered in later editions. I can't say for sure since I only own one edition (3) but that's what some folks say. I'd personally like to have the earliest one made.
buy the fourth if you can get it. it is expanded, I cant remember weather the english watch section or the wrist watch section is the new one now, but remember being told that that there was a lot of extra useful info in the new one when i got mine. if the price is right tho, cant have too many books!