Hand held v not hand held
I started with a 70 300vr f4.5/5.6 - no problems hand held
Then the 300mm f4 (it's none VR), good hand held but you need a little practice
I have never been successful in hand holding the 300mm f4 with TC14ll, or the 300mm f2.8 with or without TC's ……..I have tried the 500mm f4 and the same goes ….. I would think the 200mm 400mm f4 is the same - you need a tripod, monopod or something to rest it on ………. they severely reduce your mobility
So guys will say that they can hand hold everything
It is a very big change from a 70 300mm zoom to 500mm prime, or even the 300mm f2.8 ………. you have to live with this and get to know each lens …… they can take some time to get used to.
Also FOV ….. BIF are OK with the 70 300mm VR - flexibility becomes a problem with primes ……. and even finding the bird can take valuable seconds with a 600mm
I was thinking about a 500mm f4 …….. but I will still with my 300mm f2.8 with and without the TC …….. and I think that if I were to buy another long lens it would be the 200mm 400mm f4 for general Wildlife photography
Good luck with your decision - a new lens can take some time to get used to but once you have "cracked" it you will see it's virtues, (I hope)