Hey,

First you need to upgrade payplans to 3.7.1 version at your site. You can get installer for payplans 3.7.1 from dashboard. After that you can upgrade to payplans 4.

Before upgrading to payplans 4 , please read transition doc first.
https://stackideas.com/docs/payplans/administrators/welcome/transition-to-payplans-4

Note : please backup your entire site before upgrading to payplans 4.

Let me know if you have query.

Thank you for understanding!