Best way would be to create a large cocoa bean farm and then selling them for a fair bit of money.
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This is probably the only efficient way that requires work to earn money.
You should also have some mob droppers by now. I like mob droppers better because once they are built they do not require any actual work.
The Fractal Cake cocoa farm was an impressive build but I view that as a big investment in time that creates a benefit only in the short term. As soon as you get melons, you have little need for a cocoa farm.
To explain further, this video will also work just take longer. Zombie spawners are useful but when using the blot out the sun quest it will be hard. To automate youll need hoppers, which you wont have at island 4. You could sit there and afk, but assuming that you dont have the time, this is almost pointless. You can automate the spawners using cactus, but some of the drops will get destroyed due to the cactus. So the only efficient way to do this is the old way, and thats use the sword. And since you'll have to farm for hours, you'll need to think about repairing the stuff too. This includes, sword, pickaxe, and axe. Axe will need to be used for chopping (duh) assuming you don't have enough wood for sticks. Same thing for pickaxe, youll need to mine cobble as this is the best resource you'll have at the time, assuming you don't have enough cobble. XP you wont need to worry about, as you get from zombies.
So in conclusion, the cocoa bean farm is the best way to go.