Sorry, I'm not sure I follow your logic here. Can you explain what you mean by all gene sequences being calculated?
What I think you've calculated here is the number of all possible sequences consisting of 4 base pairs. I.e. the first position can have 4 options (A on the leading strand paired with T on the lagging strand, or vice versa, or C on the leading strand paired with G on the lagging strand, or vice versa). Same goes for the second, third, and fourth base pair. So there should be 256 such possible sequences of 4 base pairs.
But since a DNA sequence can be of variable length, and is generally longer than 4 base pairs, your calculation doesn't cover all gene sequences -- unless I'm not understanding your question.