Still about 10mm. The angle effect of 4 degrees will have a minor effect on length, and a bigger effect on the bar height. The 80 degree stem drops the bars about 5mm.

To figure all of this, assume a 73 degree head tube angle. The stem angle is 84-73 = 11 (relative to the ground).

The horizontal length of the stem would be cos11 x 90 = 88.3. The height would be sin11 x 90 = 17.2. With an 80 degrees stem it's cos7 x 100 = 99.3 and sin7 x 100 = 12.2. That makes the length difference 11mm and the height change 5mm.