That seems more redundant than strength and constitution, if I'm being honest. Strength is active, while Constitution is passive: I'd compare it to the active Charisma versus passive Wisdom.
So if I want to split Dex, how important is it to come up with a 4th mental one to keep the symmetry? Or does going beyond six already nuke the D&Dness? Or would having to do the character generation with 8 be annoying?