Here is a comparison of these popular PM tool steels in production folders.
Special thanks to Darby for letting me borrow his Cruwear Millie for so long.
I'm missing the 4v but I owned one in the past and currently make knives from it. In a production knife from spyderco it will be softer in hardness than the Rex45 but may out cut rex 45 if the 4v is at sufficient hardness due to having the advantage with slightly more carbide volume and more harder Vanadium Carbides, yet similar Strength and stability, yet on the softer thicker end may have more obtainable raw toughness at lower hardness depending on HT and Geometry.
Here is M4 vs Rex45
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M4 vs K390
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More M4 vs K390
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