A milestone by definition is a 0 duration activity- it is not an event in itself, but signifies that other tasks have been completed or that a point has been reached. Even the help within Project indicates this.

I cannot see how a 4 day activity should be marked as a milestone. If you need a milestone at this point, I would suggest inserting a new activity after the 4 day event (call it "xxx complete or whatever is significant), and link the 4 day activity to this, with the successors being transferred to the new milestone.

If you look under format (on the main tabs), choos format/ bar styles, you will see that a milestone is shown as a diamond- a 0 duration task.
A new item would establish the milestone, without affecting th eprogramme.