If you skipped Course 2 and came directly to Course 4, you missed the actual “intro to TF” here. That was given in Week 3 of Course 2.
But I agree that a lot of people feel that the amount of background they give in TF is not really as thorough as it should be. There are lots of TF tutorials out there (“google is your friend” in more ways than one 
) and here’s a useful thread on Discourse from a fellow student who gives a much more thorough explanation of how to use the Sequential and Functional Models in Keras.