If you set up a new property, you'll use Google Analytics 4 by default. On the other hand, if you're still using the old version, you'll need to make a transition by configuring both implementations simultaneously.
It's advisable to create a new GA4 property as soon as possible, as it's impossible to import data from the old version. It's best to start collecting data and familiarizing yourself with the tool before completely replacing the old version. Google recommends this transition method.