David Stephens , Apr 19, 2011; 10:44 a.m.
I've owned the 400/f5.6 and currently use the 500/f4 as my birding lens. The 400/f5.6 is fast focusing and sharp, but really needs IS. In your price range I'd go for the 300/f4 IS with the 1.4x TC, yielding 420mm of focal length.
When you get into these super-tele focal lengths, IS becomes very important. Even with my 500mm, I shoot around 90% handheld. Others use a tripod, and I did for many thousands of clicks, but I find that my keeper ratio for birds in flight is much, much higher when I hand hold. With the IS, I can successfully hand hold down at 1/500th second, where without IS I'd be wanting at least 1/1000th.
The IQ of the 300/f4 plus 1.4xTC is going to be very close to the 400/f5.6 when mounted on a tripod, shooting at a newspaper on the wall. In the real world, chasing birds and animals through the woods, in all kinds of light and then cropped down, there want be a iota of difference in IQ. Looking at flat test patterns at 100% is useful information, BUT it doesn't reflect real world performance, handholding while shooting a fast moving subject.