Quote from MegaTrain
So I finally got an answer regarding the slime spawning issue on the 4.0 map:
The crazy thing is, I'm POSITIVE I saw LP videos where people had tons of mobs spawning on their island when they left it at night. Maybe I just always saw that on videos of the 3.1 map.
You are right, I never moved the spawn point in 4.0 and have move it in 4.1.