I don't do weddings, heck I'm not even a real photographer, so bear that in mind when reading on.
The f/2.8 might get you faster focus (it passes more light than an f/4), and allow you to get a shot in less light than the regular (non-IS) f/4 (1-stop worth). It is expensive, but as I recall, the pricing for the two lenses you are considering is roughly the same. Weight is also a consideration. I have the 2.8 IS, and it is heavy.
I have seen (somewhere in the past) that the f/4 version is also a tad better optically than the f/2.8 (this was before the release of the f/2.8 IS II which you are not considering anyway).
The most important advantage of the f/4 you are considering is its IS capabilities (supposedly 4 stops worth). Lets you handhold with slower shutter speed than a regular lens. I think that one feature would seal the deal, particularly if you have to take a lot of your shots hand-held..
http://www.the-digital-picture.com/R...ns-Review.aspx