Power Lens will add 4 Sp. Atk to any Pokemon defeated (not just ones that give out sp. atk EVs), and since you're fighting Pokemon that give 1 Sp. Atk EV each, you're getting 5 EVs per battle, not 4.5.
A Careful nature does not mean you're get 4.5 EVs per battle. What the Careful nature means is your Dialga's sp. atk is 10% less than what it should be if it had a neutral nature regardless of EVs, and EV gain is not affected by natures.