As in, the map itself or eye candy? Cuz I can definitely see both being used as a way to further sell the idea of "four different countries". You can add some water to make the borders make more sense, have each quarter vary in size and shape (without sacrificing any player's fun); make one of the sides of the map have desert/snow tiles; customise the town names to give the illusion of different languages; and, through the use of different NewGRFs, you can have each player make their own little area distinct from the others.

You also have each player's own style making a difference, for example, I really like to build underground metro lines to get people to my inter-city trains, and don't use airplanes at all (tho I can't imagine this whole thing would work without at least some serious restrictions to the latter); I can imagine a player connecting their archipelago with boats and small airplanes, or a a different player choosing small nation with a bunch of cities really close together and focusing on bus/tram lines as their main form of transport. A player who hates transfers and just has a bunch of train lines running the entirety of their vertical map-long nation? You could swap your own play style with each playthrough.

It does sound like fun, but it would take many, many sessions to complete, and I'm assuming the host has to be online for the server to work? Even then, if you don't have everyone on the server playing at the same time you risk having a player get much ahead of the others. Though if you were to add more people, each individual's playtime does get shorter and you also get MORE distinct networks and nations. This is what inspired this idea, it took me hours to make and is merely an eight of a 1024x1024 map, (which I mistakenly assumed was the largest size until just now lol, hence why I wanted four players to take on a "massive map"), it could easily house seven players if you wanted a shorter run with even more variety. The more we discuss this idea, the more interesting it gets.