I had the 100-300 f4 for a while and liked it as a lens, however I found that there was a noticeble softening when used with a 1.4x tc. I also found that the AF slowed with the 1.4x on, that said it was stil faster than the 135-400 (a friend had one so I have done direct comparisons). My only slight issues with the lens was that it was prone to hunt in low light and the images were sometimes a bit flat (it seemed to lack the contrast that my other lenses had).
In the end I got rid of it and replaced it with the 70-200 f2.8 - this lens handles low light better and the focus is very fast. I found that the 70-200 f2.8 with a 2x tc gave better results than the 100-300 f4 with a 1.4x.
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