I've actually carried both the 800mm PF and 400mm f/4.5 on a solo birding walk/hike of around 5 miles. I carried them both on Black Rapid straps so I eliminated the weight of a camera bag. The 400mm f/4.5 weighs almost nothing and the 800mm PF on a camera body is pretty light duty. I had a 1.4 TC in my pocket.
My thought process was there is a clear difference and no real overlap between the 400mm and the 800mm lenses. I considered the 400mm lens for large wading birds and environmental images and the 800mm PF for small birds and somewhat distant subject.
These two lenses were a lot easier to carry than just the 600mm f/4 by itself with a tripod. I'm not shooting the 600mm f/4 handheld.
My thought process was there is a clear difference and no real overlap between the 400mm and the 800mm lenses. I considered the 400mm lens for large wading birds and environmental images and the 800mm PF for small birds and somewhat distant subject.
These two lenses were a lot easier to carry than just the 600mm f/4 by itself with a tripod. I'm not shooting the 600mm f/4 handheld.