The coordinates of two points A and B are (1, 3) and (9, -1) respectively, and D is the midpoint of AB. A point C has coordinates (x, y), where x and y are variables.

1. State the coordinates of D.
2. Given that CD² = 20, write an equation relating x and y.
3. Given that AC and BC are equal in length, find an equation relating x and y and show that it can be simplified to y = 2x - 9.
4. Using the results from parts (ii) and (iii), find the possible coordinates of C.