I would also agree with area restriction in the form of certificates to avoid any backfire from the "this is sandbox and you should be able to roam freely" crowd, whether it's fame, combination of fame and quests, or anything else that needs a relevant amount of time to achieve, it should be shown as acquired certificates to access that will be requested by gate npc guardians (so the gating can have some sort of lore behind to justify), on top of it a considerable amount of guardians will roam the lands, if you are afk within a certain time to show the certificate on demand (which could just be a click to confirm or a more complex randomised position/color/texture procedure to avoid bypass by visual recognition based autoclickers), they will attack you.
Not sure if that's the main reasons for them this is case, but I presume that zoning is necessary to limit lag issues due to huge amount of clients to compensate at the same time which would be huge with a lot of player in the same world. In other games that's possible with optimisation, faster and more hardware and separation of Albion Online Power Leveling into channels to limit the amount to compensate within a group of servers (Channel). Here the other way around, cut the seamless world but keep only one channel. They could have done one seamless world but once they would reach the cap of concurrent users which would be way closer than the whole population, they should have closed registrations, clearly doesn't work.
The 4 entrances makes things easy for them to connect and strealine the process of map creation and connect the dots.