You are in a good position.
Let me see if I have this right: xyz.com is like the mother site of four regional sites. This is a common situation.
I have to make the assumption that the four sister sites basically contain the same content.
When I run into this situation I consider the five pieces as being part of one website. The mother site is like the landing page and the sister sites are various sublevels.
Looking at it this way, how would you optimize it? What would your internal linking structure look like? Would you want the sisters sites to have the same content, the same Titles, Descriptions, Headings, paragraph text etc.?
I don't know if that answers your quesion but, as I watched your post go unanswered for 12 days, I hope this helps.