From everything I've seen, the 300mm f4 IS is optically inferior to the older non IS version.
This seems to be true. I hate to say it, and won't say I told you so, but several people have bought that 300/4 IS recently, and either said they weren't happy with it, or I can see it in the images they post from it. It's really pathetic of Canon to sell this kind of equipment as top of the line gear, and you well know my views on the 100-400/5.6 IS. Yes, there are always going to be a few good copies of any lens, but that does not condone putting the bad copies out on the shelf with the few good ones either. I have been there, done that, and it's a terrible feeling to know you just got fleeced.
And now, down the line some more since I wrote the above quote, I do find IQ degradation when using the 1.4x TC. It's not terrible, but for one who uses it both ways, it is noticable. I have taken to using the 300/4 naked now when in my yard and the birds are close and am shopping around for a new 400/5.6. I wish I had never sold my old one. 