Can I transition a 12 mm to a 8 mm laminate wood floor in an adjacent room?
4 mm is between 1/4" and 1/8". With a transition I doubt it would be very noticeable. You can add 1/4" (6.35 mm) underlayment under the new laminate and that would reduce the difference to 2.35 mm which is between 1/8" and 1/16". Of course the new laminate would now be the higher one, but the difference in height would be reduce in half