43 mm should be fine. That's the standard width for a P-bass nut...and unless you special order it's the width for all PB 4 string bass nuts, IIRC.
The US nut is supposed to be 1 11/16"...which is just under 43mm. Generally you want the nut to be just a hair wider than the slot so you can sand it down to exact dimensions.
...but at 42mm I'd be a little worried that it's too narrow.
If push comes to shove you can buy ANY bone nut that is bigger than your slot and sand it down to size. Keep the plastic nut and use it to know where to put the string slots.
I have made nuts from bone blanks, which are very cheap. It's a ton of sanding, but it only takes a couple of hours tops while you listen to a podcast or audio book ;-)
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