This means she is pregnant, right?
Yep. But who's the father, František or Matěj? ;)
Gotta be Matěj. We learned in another exercise that "František je chlapec, kdežto Matěj je muž." On the other hand, I don't think we know František's age...
Does it have to be pregnancy? Could it also mean her fourth month of, for instance, a new job?
Very unlikely. It would have to be said explicitly.
Okay, thank you.
"Kateřina is four months pregnant." Acceptable translation?
why not "Katherine is in the fourth month?"
Do not translate names.