Below you see the 200mm Arca Swiss plate attached to the lens foot. This version of the 400 has a tripod collar that is one piece and therfore does not have longer aftermarket handles.
Looking down the barrel without the grip on.
I've put a 200mm length Arca Swiss adapter on my Canon 400mm DO f4 lens. Its shown here with the 2x extender mounted.
This long Arca mount allows you to balance the lens when mounted on the gimbal mount I use.
But what I found that when handholding the lens is the arca plate is not very ergonomic.
Slip a silicone pot handle holder over the plate, and its much more easy to grip.
There are less chances of the lens slipping from the fingers. The silicone sleve is very grippy on the inside and outside.
This enhancement is cheep and easy.
Order the flat grips here...Amazon
I got the 200mm Arca Swiss bar that just fits the grips as you can see. This one worked well...Amazon
You could use a shorter plate and cut the pot handler to length. For me the 200mm length just clears the lens.
Happy photography!
