Block of 4 measures 51mm x 45mm. While I don't have a block of 4 in a mount, I do have an entire open 15-stamp booklet mounted. Height of the mount is 50mm. This pretty much agrees with past experience that the mount usually ends up measuring about 5mm more than the actual dimension. So I would guess 56mm x 50mm mounted. But depending on how closely the block is trimmed at left & right, you might need to add a couple of mm to the 1st (horizontal) dimension.