I dug into the detailed data behind Google Analytics 4 and compared it to the old Google Analytics we have come to know, and something popped out that was unexpected: sometimes the city being recorded for a specific user’s visit was different.
I did a detailed comparison over a period of 5 days, aligning user activity between the Big Query export from GA 4 and the GA User Activity API. Thankfully the two systems appear to use the same user id values. It confirmed that there were a number of cities that were recorded differently. Here are a few examples:
It appears the two systems use a different algorithm to determine location. Keep that in mind if you are transitioning from one system to the next — any geographic-based filters or segments may need to be altered.