
Originally Posted by
Charles Scheffold
It's like comparing apples and oranges - the OP wanted to know about the 400/5.6, so comparing to the 500/4 isn't completely fair. The 400/5.6 is designed to be a lightweight, fairly compact telephoto lens at a price point that is affordable. The 500/4 is bigger, heavier, and 5x more expensive.
It all depends on context - I use my 400/5.6 for casual hikes up large hills/mountains. Would not want to drag my 500/4, tripod, gimbal, etc. on those hikes just in case I see something :)
Charles