Since question of what and how to approach this appears so often . . .

The quoted material below is from a short book by Brooks Jensen, editor of LensWork and appeared in Letting Go of the Camera, item #16 of the chapter titled 'What It Means to be an Artist'.

" .. you will be routinely asked by your friends to photograph their weddings. Early in your career you should agree to do so, one time. Show up with your 4X5 camera, clunky wooden tripod and plenty of sheet film holders. Shoot several Polaroid tests and then ask the entire wedding party to hold still for a 45 second exposure at f/64. Explain to them that one of the advantages to working with large format cameras is that you don't have to work with cumbersome lights. If this does not suffice and they still look fidgety explain to them the concept of reciprocity failure. You may then attend all future weddings as a mere observer."

Unfortunately there is a growing number of "professional" wedding photographers out there who visit Costco and get a cheap kit setup, and then stop off at Office Depot to get cards made and they become an Instant Pro!
The sickening part of it is that the quality of the images is usually garbage, as can often be expected based on the seriously discounted price.
In the past year I have had 2 customers come to me to get prints made of their "Pro" wedding photos because the Kiosk at Walmart will only print them a 2" x 3" in size. When examining the CD with their wedding pics I find over 1500 thumbnails of 640 x 480 images, and that's all the client got from their so called "Pro" photographer. I am currently under subpoena to testify in an upcoming civil suit against one of these faux pro photogs over these very images.
There is probably little the scammed client can do to recover anything more than the photographers fees, and then only if the judge is sympathetic, because the photog DID present the client with images of the wedding as they were contracted to do, its just that the images are way too small to print as they are only sized for Facebook posting and nothing else.
You get what you pay for.......if you are very lucky!

With weddings, where you are hand holding most of your shots, and with the variety of non ideal lighting situations, the best lens for the job is the F/2.8L IS . . . the second best choice would be the non IS F/2.8L. The F/4.0L, IS or not, just doesn't hack the low light situations. Even if you never shoot at F/2.8, at F/4.0 the camera will have issues focusing. Remember that the camera focuses with lens wide open, and F/2.8 will let the camera "see" at much lower light levels.
As far as sharpness is concerned, all of the "L" lenses are more than adequate and unless you are a pixel peeper, you would never see the difference with the naked eye.
The suggestion of the older version F/2.8 IS is a great idea.