According to the principle of alternation the 2th and the 4th corrective waves within an impulse wave will alternate (in form and depth)

For example,  If the 2th  wave is a simple corrective wave (e.g. zigzag) the 4th wave is likely to be a not simple, but complex corrective wave (for example, triangle).

If the 2th wave has retraced 50% of wave one, 4th wave will probably retrace only 38.2% of wave three.