My stepsons were in a 4/5 split last year (1 in 4, 1 in 5, LOL)
It was fine, honestly, to me it seemed like it was a way to allow the teachers to "specialize" more like in a middle school setting, where you actually change classes? Because, in the split, 1 teacher would teach likemath and another would teach social studies, etc. And then they could split and have like a higher reading group and a lower one, etc. I can't really think of any "cons" versus a "regular" classroom.
Like a pp said, sometimes it is because they have like, 2 full classes already of each grade, and have like a handful of students in each grade leftover, so they thrwo them together into a "split" classroom, because they can't afford to hire TWO more teachers.
the only thing I would worry about is if the class your daughter is taking is sort of lumping everyone together and NOT taking different abilities into account. For example, if she is in grade 2, and very bright, but most of the class is grade 1, and they are mostly doing grade 1 level stuff, your dd might get bored, etc. However, on the other end, if she is maybe struggling or would need/enjoy a slower pace, then perhaps haveing the opportunity to be with some younger kids would be good for her, etc....