I would trade my 70-200 f/4L IS for a 100-300 f/4L IS
Based on other posts, this may put me in a minority, but I'd seriously consider selling my 70-200 2.8 for a 100-300 4.0. My 70-200 spends a lot of time with the 1.4x on it anyway, and a 100-300 would complement my 24-105 4.0 very well. Performance of the 2.8 zoom with the 1.4x is very good, but I'd love to take it out of the optical path.
That's a nice example how different photographers can be.
Allthough I do own a 1.4x TC, I've never used since I bought the 100-400. I take the 70-200 f/2.8 whenever shooting in low-light conditions where I'm always happy with every f-stop I can get, so the TC stays off the lens.
And personally, while I'm very happy with the IQ of the 70-200 f/2.8 IS, I'd grade the combined quality of the 70-200 + TC 1.4x II as "OK", but not as "very good".