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Hyper Diamond Lvl4 is obtainable from the Jewelry Store Robbery, in order to get a lvl4 one, you need to have a lvl3 hyperchrome in your inventory.
With Diamond LVL5 peaking in value, and slowly dying off, Diamond LVL4 as a result has also lost a bit of demand. While it still performs quite well, it struggles to get the same kinds of offers it used to beforehand, and its nowhere near as good compared to other above average demanded items, therefore a demand drop is necessary.
Supply is widely distributed — no single holder owns enough to register as a hoarder.