F/4 is fine for almost all users. I'm probably one of the few who actually use f/2.8 on a regular basis, just because of the lack of light in theaters. Now that I have my 24-70 MK II, I do not use wider very often.
I'd say this is one of the new family of Video optimized lenses with IS that Canon is developing. They see video as a big selling point, so we are going to see more video features. Who knows, if they get good enough for me to use, I might take up video again. I did do it on Super 8mm film in 1968 for a few years, then again in the lete 1980's and early 1990's on SVHS. Using the video editors with my Panasonic Industrial recorders was time consuming to a extreme. Programming the controllers, black-bursting tapes - YUK!
I've also done it more recently using computers to edit, and even with my DSLR's, but I'm not happy with the results, and not willing to invest time and $$ into video, at least for now.