It appears to take the tablets about 5 minutes to dissolve completely (I watched once), so that's the amount of time you'd save. You might lose some disinfecting capability if the tablets aren't put in the water immediately after being crushed.
The 4 hours is for cloudy, cold water at 40* F, sort of a worst case scenario, for cysts of giardia and cryptosporidium, especially the latter. Nobody (I asked) is willing to say how much less time is needed if (1) the water is 40*F but clear, a more logical scenario in the mountains of the western US, and (2) it's a warm day so the water has warmed up to 50-55*F at the end of 2 hours, a logical summer scenario. Undoubtedly nobody has ever tested at that level.