you can squash together the last 4 commits; since you committed them locally. To do that you would edit the file to:
pick 8f7fe87 ok
s c80fad5 ok
s 479d27c ok
s b6ca869 ok
after that you will be able to change the (combined) commit message.
However this will not squash them on the last commit with "master commit message". The problem is that your "master commit" has been pushed to the server; so you cannot simply append commits to a commit that has been pushed to the server.
That commit has a certain "hash" code; and since other users might have pulled that code; git does not (or not easily) allow to append to a commit that has already been pushed to the server. If you want to do that, you can see this question