Clearly not what to expect from a 100-400.
Here's how I tested 1.4 teleconverter on my 400 5.6 lens.
(I knew the 400 was good)
Bright sunny day, camera resting on beanbags, on a concrete wall.
Concrete wall next to road that gets very little traffic.
I'm confident to not shake camera when pressing shutter, but took a few using 10 second timer anyway. All pics were great. No surprise, I ruled out every possibility of poor result, external to equipment.
If you can get good results without being as thorough as I was great, but if you can't, be thorough, it's not that hard.
Then get some beautiful hand helds like above, if you can
There is no tripod / head combo in the world as solid as this: