Nowadays yes it's worth. Current 400/2.8 is really light (more in old 500/4 range, which means you can work from hand), while old one was extremely heavy, and more or less useless for everything but sport, where you can bring your roller on sidelines. As soon as you needed to carry that beast it was just too much to bother, and 400/4 DO was perfect option. But then again, at that time prices of 400/2.8 and 400/4 DO were more or less same.
But it all depends what you need this lens for. For shooting sport under lights, 1 stop will make huge difference. For shooting sport outdoor during day, f4.0 is no big deal (afterall I, and everyone else, easily manage to do everything with 500 and 600mm lenses which are all f4.0).
As far as new 200-400 lense goes, I was not really so impressed when I had it for day back in February (when CPS had it on skiing World championships). Sure zoom is nice, built in converter is on one side ok, but then again it really doesn't cost much to have converter in pocket. And I'm scared to even think what will be price of this lens once it comes out officially.