I don't think a 4x4 screwed to a 2x4 would be stable enough for a hoof stand. You need a bigger base for sure.
Mine is the base of an old diner pedestal table, cut off to the appropriate height and then MDH did some fancy stuff for me with it. But really, just the pedestal would work with protection on the top of it for the hoof to sit on. Maybe a piece of a 4x4 would work there -- drill a hole the size of the pedestal in the block and use construction glue to attach it to the top of the pedestal. That will work instead of your knee if you like.
However, you'd absolutely need to learn how to hold the hoof with your legs unless you had a cup on your hoofstand.
I would ask your farrier to show you how to handle your horses feet this way and maybe she would also be willing to show you how to rasp just a touch off in between her visits. Maybe you can get strong enough with some practice. Try just cleaning the hooves while holding the hoof with your legs at first.