I completed skirmish 4 all 3 stars. It it says I have now completed it. Did I miss something
I've not seen this situation before - but it's talking about a skirmish. They usually refer to the land battles you have to have on your turf to clear area for building. Perhaps you haven't cleared one of these?
A skirmish and a chapter are different things. You have completed Chapter 4 of the Hero Chapters.
Skirmishes are the PvE battle you fight inside your turf, you need to complete Skirmish 5 and unlock the 2nd batch of Manor/Barracks/Infirmary spots.