Do you really say that that extra stop is more important than a sharper lens? Could you explain why?
The f2.8 aperture has, for me, two main advantages.
1.) Depth of field. Sure f4 will give you some subject isolation, but f2.8 just gives you so much more. Sometimes if your shooting a subject and the background is either really close behind, or really distracting, you need to blow it out completely, and setting the aperture at 2.8 lets you do this so much more than an aperture of f4.
2.) Twice as much light. Say you're shooting a dark concert and you know you need a certain shutter speed to freeze the motion of the band, say 1/500 of a second. At f4 you may need an ISO of 3200, which on most cameras is pretty noisy. With an f2.8, you can lower your ISO a whole stop to 1600, which is suddenly very easily usable on all DSLRs. This works too by the same principals for sports where fast shutter speeds are needed.
Thats why Id recommend an f2.8 over an f4 any day. Even if the f4 has some sort of stabilisation, that doesnt help in either of the above situations.
Does that answer your question Ruben?