Unless you are proposing that CS schools are not linearly rankable or that being "top" is not a unitary quantity, then the only way that I can see that the set

Unless you are proposing that CS schools are not linearly rankable or that being "top" is not a unitary quantity, then the only way that I can see that the set of top US CS schools has a cardinality of four is if you are defining that axiomatically.