Identify At Least Four Key Milestones In The Development Of Cognitive Psychology As A Discipline?

Technically, this can be answered more than one way.  Jean Piaget originally pioneered clinical work for cognitive psychology, which strengthened its position as a discipline in general.  However, at the time that he was championing its development, there were a series of discoveries regarding methods of research that had a dramatic impact on existing psychological forces.

Application of the scientific method (still debatable as to whether "scientific" is accurate), experimentation, catharsis, social learning theory, sample elements, and the understanding of inter-observer reliability were all critical components for psychological study.
Directly related to cognitive psychology in later years was the discovery of cognitive maps, place-learning, and negative//positive transfer of learning, which were all instrumental in increasing our knowledge of cognition and its application to the environment.