A 4/5 for my personal scale is a class that functions as written and has some cool features that makes you want to play it. The reason it may lose a star is becuase it may have some options or abilities that don't entirely fight the tight balance of 2e. In the case of the Theorist, there are some abilities that a GM may want to fine tune or otherwise adjust if they allowed the class at their table. Overall though, the concept is solid and it will definitely attact a set of players that like the idea of copying spells from their foes or being the premiere counterspell spellcaster.