Two names for the same split
According to Google Analytics Help, bounce rate in GA4 is defined as "the percentage of sessions that were not engaged." Flip that definition over and you get engagement rate: the percentage of sessions that were engaged. There is no scenario where both numbers move independently -- they always add up to 100%, because every session is classified as one or the other.
What makes a session "engaged" in the first place
The classification hinges on one definition, and it is worth knowing exactly: Google Analytics Help counts a session as engaged if it lasts 10 seconds or longer, includes a key event (GA4's term for a conversion), or includes 2 or more page or screen views. A visitor who opens an article, reads for 15 seconds and leaves counts as engaged even though they only viewed one page -- a meaningful change from Universal Analytics' old bounce rate, which counted any single-page session as a bounce regardless of how long the visitor stayed.
Why this replaced the old bounce rate entirely
Universal Analytics' bounce rate had a well-known flaw: a visitor who read a 2,000-word article for four minutes and then left looked identical to one who clicked away after half a second, because both triggered only one page view. GA4's engaged-session model, built around actual time and interaction rather than page count alone, was designed specifically to separate those two outcomes. Google retired Universal Analytics for good on July 1, 2023, which means bounce rate as most marketers learned it no longer exists in any actively supported Google Analytics product -- what is now called bounce rate in GA4 is the inverse of this newer, interaction-based engagement rate, not a continuation of the old metric under the same name.
Which number should you actually look at?
Use engagement rate as your default, for one practical reason: GA4's interface, default reports, and benchmarking language are all built around it, so you will see it more often and compare it more easily across your own pages. Bounce rate still appears (it is one click away, and some people find "lower is better" more intuitive than "higher is better"), but it carries no information engagement rate does not already carry in reverse.
Where either number genuinely helps a small business is in comparison, not in isolation. A blog post sitting at 40% engagement when your blog average is 60% is worth a look at its opening paragraph and load time; a landing page at 30% engagement that still converts at a healthy rate may simply be doing its one job quickly and is not necessarily a problem. Read engagement rate next to conversions and time on page, the way we cover in engagement rate in GA4, rather than chasing a higher percentage for its own sake.
A quick gut check before you act on either metric
Before treating a low engagement rate (or high bounce rate) as a problem, rule out the boring explanations first: slow page load, a traffic source that brings the wrong audience, or a page whose entire job is to answer one question and let the visitor leave satisfied. See the website metrics that actually matter for how to weigh this number against the rest of your dashboard instead of reacting to it alone.