Well, the working hours per day are one constant, and the crew count is another. Four shifts will involve just as many man-hours as three - it's just that some people will have to participate in two shifts when they previously did only one, or in three when they previously did two.
It's not as if we would have seen our main heroes stick to just one shift anyway. Even outside emergencies (when one might assume that people from all shifts are beaten to general quarters) we saw different combinations of heroes awake when, say, Riker or Crusher flaunted their pajamas to the camera.
What would be won by more shifts would be shorter
shifts, chiefly - potentially increasing alertness and OTOH giving somewhat greater flexibility in responding to casualties.