Actually, many concert guitarists (and others) consider 4mm just about standard! That's not me.
While the primary purpose of adjusting the truss rod (which I assume your guitar has) is not to change the 12th-fret action, as you may know, it does have that effect. Since that is less invasive than sanding the saddle, and since it's a reversible process, I would certainly start with that and see how much it affects the action (assuming this will result in an acceptable amount of relief). It may just be enough.
-Bob
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