I wouldn't use 4" wheels if you got a die grinder (unless you can control the speed). If it's 10,000 rpms +, that's _way_ too fast for a 4" wheels. Your fpm would be about three times recommended speed, plus the wheel is likely to fly apart. See if the max rpms are marked on the wheel.
I use cart rolls or small drum sanders in a grinder. For 4" wheels use a drill or flex shaft at slower rpms.
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