what are the differences between teaching approach, teaching method, teaching techniques and teaching strategies? 

In my opinion, the TEACHING APPROACH  is the most general, it involves all the assumptions, focuses and aproximations written by different authors on different books about language teaching.
when teachers read those assumptions, each of them establish their own METHOD or ordered procedures plan to teach  English, that is why it is said that a method is based on an approach. With a established teaching method, the teacher can already disign an overall plan related to teaching. This plan has to include specific classroom activities that the educator has to perform with learners, using some skills which are called TECHNIQUES. And TEACHING STRATEGIES refers to the ways to carry out those skills in planning.