At long last, after much thought, I present the map. The challenge was coming up with something that was recognizably a transition form Sosaria to Britannia that would also actually work as a game map. My solution was almost entirely algorithmic, the main exception being adding a narrow channel to Britannia Bay to prevent it being an inland lake.
There’s a line in the U4 manual about landmasses rising and falling, so at the point of this map the main recognizable landmasses from U4 have risen but the old ones from U3 haven’t completely fallen. Or to put it another way: with a 128×128 map it would be a shame to give a quarter of it over to open water.