Four positions of one die are drawn. Which number is on the face opposite the number that is on top in position (iv)?

The top in (iv) is 4. 4 is seen with 1 and 5 in (i), with 3 and 5 in (iii) and with 1 and 2 in (iv). Its four neighbours are 1, 5, 3 and 2, so the face opposite 4 is 6. (3 is opposite 1 and 5 is opposite 2.)