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...
...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.
You realize that having people work double shifts to fill out a 4 shift rotation is essentially just running a 2 shift rotation?
Adding another shift doesn't add any man hours, in that you're correct. lets take for example a shift rotation of 3. You have 3 crew to work that rotation. now your boss asks to add a fourth shift. You have 2 options. Somebody works a double shift or you add another crew member. The former is a scheduling nightmare if you want to keep it fair to the entire crew and also means you'll have a shift that is more exhausted at the end than others. The second option requires another person that needs to be accommodated.
Another option is that you use efficiencies to expand your available man-hours. Say, you have 2 people manning consoles, that could be handled by one person running both consoles, since there is wait time involved for processes to run (IE: Fire off one search, or diagnostic, and while it's running go do soemthing on the console, etc). This one change just gave you 3 extra people for that extra shirt. I could imagine there are alot of these cases, since they're not worried about having to save on expenses and you wouldn't want to run your crew at 100% efficiency all the time and filling every minute of their time with workload, or they would get burned out.
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