I bought the CS4 master collection for windows and I change my computer to a Mac. How to upgrade my license?
There is no option to upgrade your CS4 license except to subscribe to the Creative Cloud's special first year discount plan for CS owners (Adobe: Creative, marketing, and document management solutions).
Platform swaps are only allowed for the most recent released CS software (CS6). You cannot get a Mac version license of CS4 thru Adobe since Adobe does not sell older versions and the only CS version for sale is CS6... CS4 does not qualify to get CS6 at an upgrade price.
You can purchase CS6 thru the following page: http://www.adobe.com/products/catalog/cs6._sl_id-contentfilter_sl_catalog_sl_software_sl_c reativesuite6.htm