Can I still access my plug ins that I purchased through character creator 3 for character creator 4? When I launch CC4 it's acting like I need to re-purchase SkinGen.

you try to redeem the plugin upgrades

Open a new browser in which you haven't logged in Reallusion ever.
Or use incognito

Login into the desired account

(Make sure that it is same account that was used to purchase the plugins)

Check order history

Try this link  on same new  browser in a other Tab  (also Peter can give you another link , he has given it in other threads)
https://www.reallusion.com/event/freeproduct/default.aspx?cid=292&chk=7417146ea8b2c7a62b2b454c5d99134d&_gl=1*wejrlf*_ga*MTY2MjQzNzc1Ny4xNjQ5MTA0Mzky*_ga_Q3FS71VPKC*MTY1MjQyODY1MS4yOTkuMS4xNjUyNDI5NDUyLjYw