You’ll get this achievement for landing on any four uncharted worlds. Most of the solar systems in the game have at least one place of interest where you can dock or land, either boarding some other spacecraft or, somewhat more commonly, landing on a planet. You’ll likely earn this achievement while working on Completionist, as you’ll land on most of the uncharted planets in the game in the process of completing some assignment or another. Whether that assignment is gained ahead of time in another location, or found while exploring and scanning planets is irrelevent as far as earning this achievement goes - you’ll likely need to do quite a bit of both to find enough planets to land on.