Say, if I was using a Basculin with Adaptability, and I used Aqua Tail on a Rock/Ground type would it be 5x as effective, or would it still be 4x as effective?

Well, Technically ADaptability raises the power of the attack I think, so 90 power plus 45 plus half of that times 4 to the weakness. So the power of that would be 810. Not the effectiveness. so then that would be 5x as effective so your are correct.