Even after you have created your new GA4 property or connected your existing GA3 property with your App+Web property, you will continue to see the 'upgrade to GA4' option in your GA3 property. However, when you again click on the 'Upgrade to GA4' link, you should see the option to 'See your GA4 property', which confirms that you have successfully set up the GA4 property and also connected it with your old GA property: