GA4: G- prefix on new Property ID not well documented

Perhaps the answer needed to be that the D7 GA module at the time did not support this but does now via https://www.drupal.org/project/google_analytics/releases/7.x-2.7 →

SHIROU FUJIMOTO → made their first commit to this issue’s fork.

As tomorrow is July 1st and that's the deadline, can anyone clarify whether we use the GA4 Property ID or Measurement ID? My understanding is the Measurement ID is the correct GA4 ID to use with this module (NOT the property ID).

That is correct, the measurement ID (G- prefix) is what you should be using. Unfortunately I don't think there is anything we can change here as far as the module is concerned.