I would expect eventual lawsuits against the NCAA for requiring the employees also to be students and to maintain specific GPAs/degree progress. Also, why limit them to only 4-years, there are no such limits on any other students and their employment with the universities.
There already will be no scholarship limits (replaced with roster limits), it is hard to argue that they are "student athletes" and not "compensated employees." So why the requirement of being a student, and limits on the duration they can be involved.
There are plenty of regular students who are paid by their universities and required to maintain GPAs and graduation requirements. Grad assistants, RAs, etc. I don't think there's an argument to be had there.
Four years of eligibility though, that might be open to a lawsuit.