Bit.ly recently posted regarding their migration to v4 of their API. As you know, Dynamic Signal is integrated with Bit.ly at the platform level, and it's the default link shortener used by your communities.
Dynamic Signal has already migrated to Bit.ly's v4 API and the platform is using it today. There's nothing you need to do to prepare for the switch, and in fact, the Bit.ly article has us listed at the bottom as an organization that is ready for the transition.