It probably is okay to duplicate a calculation 4 times, all other things being equal. At the same time, I'd say it's probably a sign of other problems. If you need to reuse a calculation, that calculation is part of your application's domain (using the term loosely) and should be represented there, rather than being embedded inside scenario-appropriate parts of the system.
I'm not advocating cutting and pasting huge blocks of code everywhere, but a 1-line calculation duplicated in 4 spots is probably okay.