Five Quartets takes place in the Forgotten Realms campaign setting, more specifically on the continent of Faerun. As per Wizards of the Coast canon, Faerun is a very high-magic, high-fantasy world, and one wherein various epic-level magical events routinely shape the existence of the inhabitants therein.
Five Quartets, however, is a moderate-magic, low-fantasy campaign, and thus takes place in an alternate Faerun, which creates a number of key differences. The first and most important difference is that it follows a consistent path (but not storyline) from the DM’s prior campaigns in the setting. That is to say, Five Quartets stands upon the events of a number of other campaigns, and as such exists in a Faerun unto itself.
The Wikipedia page for Faerun actually provides a more accurate picture of Faerun than the majority of the official Forgotten Realms wiki, as the official site contains a great deal of 4E material as well as specifics of the 3.5E Realms that are not present in Five Quartets due to their high-fantasy nature. The best way to determine what does and does not exist within Canvas’s Faerun is to research the different countries individually, and double check with the DM to corroborate data. This page will hopefully provide the important relevant changes, specifically with regards to politics, geography, and previous campaign summaries.
Notable Countries and City States: