"Are you saying that you cant shoot low light with a lens of f4? I don't know what camera you use but it's no problem with the D4.As for isolating the background one stop doesn't make that much difference in the type of shots I take."
Well,...yes, especially if you aren't shooting a flagship camera. But even if you are shooting a D4, there are some photos you just can't shoot as well at f4 - like football under the lights, or basketball indoors, or anything with AF and a longer TC.
I don't have the equipment to shoot friday night football, but my friends who do are often at ISO 6400 at f2.8. I've seen very little out of ANY camera body shot at higher than ISO 6400 that I'd consider a usable.