yea i'd say get the f2.8 over the 4 too. girls volleyball is fun to shoot too but i dont think that f4 will be fast enough in most gyms because the lighting sucks. i shot volleyball last week and had to use iso 3200 when i used the 70-200 f2.8 and iso 1600 with my 85L
85L @ f2 1/400 iso 1600

and with the 70-200 f2.8 1/320 iso 3200