There won’t be any ST5. Basically the major versions have been abandoned. It’s just Sublime Text from now on.
A license enables you to receive updates for 3 years. A private license keeps valid forever, but you’ll need to upgrade if you want to continue getting updates. You can however decide to not upgrade and keep going with the latest available / valid built forever.
Details can be found at https://www.sublimehq.com/sales_faq