In both positions of this die the 6 is on top. Which number is on the face opposite 4?

With the same face on top in both pictures, turn (i) so that its faces line up with (ii): the side faces around 6 run 4, 1, 5, 3 in order, so 4 and 5 are across from each other (as are 1 and 3). 2 is the bottom, opposite 6.