This guide explains how to obtain Four Corners of the Earth achievement in less than a minute.
How to Get Four Corners of the Earth Achievement
Connect Tempelhof, Alexanderplatz, Schloss Charlottenburg and Potsdamer Platz. If you're like me and know nothing of the world and its wonders - those are in Berlin.
These locations are all fairly large plots of land but are also unassuming - except for the Schloss Charlottenburg which is a castle in the middle of empty plazas.
This achievement reportedly has a bug - it has to be done in Sandbox.
Fortunately if you start a Sandbox game on easy you'll have enough money to build four helipads and buy either helicopter with a little cash left over.