I shoot about 75% sports (mostly mountainbiking) and 25% everthing else (events, some weddings etc). Often when i photograph in either situation I am shooting in low light, I dont like to use a flash so I use a big aperture and bump up the iso.
The thing about the 1d mk4 that worries me is its a crop sensor and how would image quality compare to that of the 5d mk3. The other thing that worries me is how much better in image quality is the 1d mk4 over the 7d?
I've been shooting with a combo of the 5D2 and a pair of 1D4 bodies for a while now. I'm mostly sports shooter (pro cycling from a motorcycle) with other contract work that ranges from other sports, to aerial, to architecture, to product, to travel... a nice variety.
I think it is pretty easy to say you're best bet would be the 1D4 over the 5D3.
Going down in fps will be (imho) the the biggest thing that should steer you away from the 5D3. The build quality of the 1Danything is so rock solid, and you don't have the battery door or grip interface to be a weak point in weather sealing with the 1D4. I've shot quite a few stages from a moto in the rain with 1D bodies hanging from my shoulders from the moto and personally have never had one fail yet.
As to image quality - keep in mind that you're going from a 1.6x crop factor with the 7D, so either would add better bokeh to your shots, tho - yes - the 5D3 would give you better bokeh than the 1D4, but the 1D4 is a pro body and since the 1DX still isn't shipping yet - is the big dog for Canon sports shooters... great image quality is part of that reason.
I did a comparison of a shot made with FF, 1.3x and 1.6x crop bodies, you can see the results (and my method and reasoning etc)here:http://blog.willshootphotography.com/2009/02/canon-crop-comparison-5d-vs-1d-mark-ii-n-vs-50d-bokeh.html
the differences are not drastic, but they are there. Also - much of that focus fall off "quality" comes from the glass you're using, not just the sensor crop factor...
As for the AF system - having used the 7D - the 1D4 will blow it away... your keeper rate from your mtb shoots will soar. It is a powerful and flexible system. The IQ will still work great for your wedding gigs etc...
My dos centavos would be to pull the trigger on the 1D4, either new or a good quality used one.