I'm trying to model a system in which there is a "cross" (a four-way tee). How would this be represented in CAESAR II and what would the SIFs be like?

CAESAR II follows specific piping codes. Look at the code you are doing the analysis under and use whatever SIF they have for a cross.

Of course if the code you are working under has no SIF for a cross you are responsible for whatever SIF you use.... COADE/CAESAR II does not perform fatigue evaluation of components.