I heard that the 4 tile rule doesn't apply when the two cities are on different landmasses. I tested it on IGE and as far as I can tell the 4 tile rule changes to the 3 tile rule for cities on different landmasses. 2 tiles is still too close, but you can settle a city three tiles away from another (your own, city state or other civ) if the two cities are on different landmasses. I can't see this having much impact on the game except maybe when settling for island spices as Indo.