Where is my MySQL 5.6 database data in MAMP PRO 4?

Your MySQL 5.6 database data is located in /Library/Application Support/appsolute/MAMP PRO/db/mysql56 .

Can I change the location of where my database data is stored?

No, you cannot change the location of where your database data is stored.

What data is in my /Library/Application Support/appsolute/MAMP PRO/db/mysql folder?

This directory contains your pre upgraded mysql data. When you upgrade to MAMP PRO 4 your mysql is duplicated and named mysql56. Your old mysql will never be used, and when you confident all your hosts have been successfully upgraded to MySQL 5.6 you can delete this directory.