I’ll cut right to the chase: I’ve been telling my own clients to run parallel instances of their existing GA account alongside a new Google Analytics 4 (GA4) property because in case you didn’t already know, GA4 doesn’t import any of your existing data. That may change but as of now, it’s not a guarantee so running both at the same time means that even if you simply ignore the new GA4 property, it’s still gathering data. Otherwise, they should continue using GA classic as long as possible.
I published an article yesterday at ArtsHacker that goes into a bit more detail and includes a fabulous reference with even more info.