Sounds like I should just do my own testing!
I recently got a 24-105, that's the F/4 lens I was thinking about. I know the F/4 max aperture disables some of the focus sensors that need F/2.8, but the general performance of the autofocus in that lens is so impressive that I wonder if it will offset the small aperture limitations. Guess I'll just have to find out.
To complicate matters, I sent my Sigma 50 in for a tune up last week and won't have it back in time for my shoot.
I do have a 28mm F/1.8 and an 85mm F/1.8 that could be good performers. I am reluctant to try hand holding my 135 L lens, but that could be an option.
I'll just have to set up a low light focus test and see what happens.