Becoming a Fourth-level Spell Caster was a significant point of divergence. If he was in the Dark Magic Region, he would have been able to build his own Wizard Tower and teach his own students. On the career path of Spell Casters, he would have become a highly qualified Wizard!
However, for Merlin, the benefits he sensed from becoming a Fourth-level Spell Caster was much more than these. The moment the last two Fourth-level spells were successfully constructed, a shockwave rippled through his entire Awareness. The Spell Models, which had been tilted dangerously, were restored to perfect balance and became as still as a pond.
All the Spell Models in his Awareness came together as a whole, which released an impressive level of Mind Power. With the current level of Mind Power, Merlin could even try to construct a Fifth-level spell!