I'm not sure you can say which edition was the most radical overhaul of the system. Well, unless you count first edition as overhauling the Chainmail miniature game :D But fourth edition was a big change from 3 and 3.5.
I don't know how fair it is to say this but, speaking as someone who never played World of Warcraft or any other MMRPG, fourth edition felt like trying to create as close to a World of Warcraft experience as a pen-and-paper game without getting sued. Classes got broken down into specific roles in combat and special abilities were painstakingly precisely defined.
Now I'm willing to bet there is a community out there that loved and still loves and still plays fourth edition but, in my circles, feeling ranged from fair to outright hatred. Part of the problem was that it really didn't _feel_ like Dungeons and Dragons. Spell casting was completely different, the baseline concepts of the setting were different than Gygax quirky wheel-shaped cosmoverse, and combat actions were like pushing a button.
And from a gaming philosophy point of view, it seems silly to focus on what an MMRPG can do better rather than focus in what is special and unique about both table top role playing games and D&D specifically.
But, to tell you the truth, I have a lot of found memories of my time playing fourth edition. It didn't really feel like Dungeons and Dragons and, in many ways, it had as much in common with a board game as a role playing game. But I still had fun.
A lot of that had to do with the group I played with. We would could have played (fill in the blank with whatever game you think stinks) and had a good time. Okay, those of you who chose F.A.T.A.L., you're right. We wouldn't enjoy that.
HOWEVER, fourth edition was also very user friendly with a very easy learning curve. You couldn't, simply couldn't, get as creative as you could with every earlier and later version of Dungeons and Dragons. The actions were spelled out so exactly that there wasn't any wiggle room. Which was both a plus and a minus. On the one hand, not being able to be creative is not a plus. On the other hand, that did make it easy to teach and play.
But where that really sang was for our game master. After years of running 3.5 and dealing with all of us turning into rules students who dreamed up clever character builds, running fourth edition was a breath of fresh air and relaxing. Fourth edition biggest plus as a sword and sorcery role playing game was how easy it was to run.
In fact, if someone were to ask me to run a fantasy RPG, fourth edition is one I'd consider. Dungeon World would probably win but fourth edition would be in the running.
Look, it is good when a game gives you a lot of flexibility and the ability to get clever and creative. That's awesome. But it's also good when a game is simple to play. Those two ideals don't cancel each other out. It just means that they have different goals and different audiences or situations. The real question is if they do what they set out to do well. On of fourth editions goals was to be D&D and it didn't do that well. Another goal was to be a balanced, playable, fun game and it did achieve that.
Fourth edition didn't feel like Dungeons and Dragons and I am very glad to finally be in a fifth edition game, which I like as a system much more and feels like Dungeons and Dragons again. However, fourth edition wasn't a bad game.