If you are doing weddings, the extra stop of an f/2.8 lens is almost essential when shooting in churches that don't allow the use of flash. For about the same price as the EF 24-70mm f/4, you can buy a Tamron 24-70mm f/2.8 Di VC. The reviews I have seen place it between the original EF 24-70mm f/2.8L and the EF 24-70mm f/2.8L II in terms of sharpness. The Tamron, unlike both of the Canon 24-70mm f/2.8 versions, also has Vibration Compensation (VC) which is equivalent to Image Stabilization (IS) in Canon nomenclature.

The only real advantage of the EF 24-70mm f/4L IS over the EF 24-105mm f/4L IS is the much shorter minimum focal distance that is useful for close photography of smaller objects. It is not a true macro lens, but with a maximum magnification of 0.70X, it is a lot closer than the other 24-70mm lenses and the 24-105mm.

Compared to the EF 24-105mm f/4L IS in terms of sharpness the EF 24-70mm f/4L IS is sharper at 24mm (most notably on the edges), less sharp at 50mm, and only slightly less sharp at 70mm when the aperture is set at f/4 on both lenses.

If you will use it more for the wide angle end, get the EF 24-70mm f/4L IS. If you will use it more on the longer end get the EF 24-105mm f/4L IS. If you are serious about doing weddings, get one of the 24-70mm f/2.8 lenses.