400DO:I agree with what was said by kmatthew (I never used this lens personally);
500 F/4 IS: Better optic quality than 400 DO and more range but price would be high (I used this lens in the past for over 2 years)
600 F/4 IS: If you can manage the weight then no regrets; optical quality + Reach both no match but again price would be higher than 500 (I am using this lens for past 4 years)
But if you are really looking for a lens in 400 range and $$$ is no problem then my suggestion would be just consider new 400 F/2.8 IS Mark II, with 1.4 TC the optics are better then 500 F/4 and light weight also.
