If you only want to represent a rotation there's some extra information in a quaternion, just like there is in a complex number - an arbitrary number in C actually represents rotation-and-scale where the magnitude of the number is the scale factor. If you stick to numbers with unit magnitude, given one part you can calculate the other, up to reflection. Similarly, with a unit quaternion you can calculate any one component given the other three, again up to reflection.

In general the number of real numbers needed to describe a rotation is the number of rotational degrees of freedom. This number is n(n - 1) / 2 in n dimensions, or C(n, 2), basically the number of orthogonal (2-)planes of rotation in R^n.

[0] https://en.wikipedia.org/wiki/Degrees_of_freedom_(mechanics)