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Hyper Green Lvl4 is obtainable from both Bank Robberies, in order to get a lvl4 one, you need to have a lvl3 hyperchrome in your inventory.
Green LVL4's demand comes mostly from grinders, as most normal traders aren't willing to trade for it (and when they do, its to flip to said grinders). However even grinders aren't looking forward to trading for Green LVL4, with most of them preferring to trade for other LVL4's such as Red or Pink (or even Yellow). This leaves the demand for Green LVL4 to be quite significantly worse compared to Yellow or Red LVL4's, therefore it should drop.
Supply is widely distributed — no single holder owns enough to register as a hoarder.