Thanks for your answers.
I'm aware that I'm looking at different applications and also different price tags. The reason for this kind of odd question is that I'm loving to shoot with my 70-200, but sometimes feel like I miss some reach and the 100-400 would fit in there very well. However, I don't really have any lens with a good maximum magnification.
Essentially I think that I would extend what I already know with 100-400, but would open a whole new universe with a macro lens. On the other hand it seems like macro photography needs a lot of dedication with a DSLR (tripod almost a must, special macro flash would be good, setting up the picture is much more complex than for landscape/portrait/zoo animals/ ...).
The other reason which makes me struggle is that I don't really like shooting with my 50mm f/1.4. It seems to me like it's not as sharp (at comparable apertures) as my 70-200mm f/4 and I really prefer the versatility of my zooms. So, I'm kind of biased against primes.