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Class 4 asks for something maths has not asked for before: a fraction is the first number a child meets that is not a count of things. Until now every number could be laid out as objects on a table. Half, three-quarters and 0.25 cannot be, and children who have been quietly coping by counting tend to run out of road right here.
The second change is harder to see. Portions get long enough that a child has to start learning from the textbook rather than only from the teacher — reading a page, deciding what on it matters, and still having it a week later. That is a skill of its own and almost nobody is taught it directly. A tutor who shows a nine-year-old how to read a chapter and turn it into three lines of notes is teaching something that outlasts the chapter by years.
Fractions in particular are worth getting right rather than getting through. Almost everything in later maths, from ratio and percentage to algebra, is a fraction wearing a different hat.