I would not recommend a Tunze Stream on a 20x20 tank. I don't even think Tunze recommends that.
If you want 4 outlets and no powerheads, I would use a at least a Mag 12 but probably a Mag 18. After all the head pressure and the splitting, you should still get pretty good flow from each outlet.
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32"x32"x20" Cube-ish tank
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