Katrin is the substitute for Sandra during the following 4 weeks

I would use the second 2 translations more naturally.

would be even more natural.

Yes, Trilingual, or even better: "filling in."

True, although I think in the UK we'd usually say "standing in". Then again, I'm not too familiar with terminology re. secretaries in particular.

@ Qmike

taking over or taking someone's place sounds rather permanent don't you think? even if you put in the time at the end. not too sure. just sounds rather final to me.

I agree with
abc.

another option

I think "cover for" sounds like something that would not extend beyond a single afternoon.