The chance to kill (CTK) on a strike depends on the difference between attack and defend, and not the percentage by which one value has been reduced.

v0 cmaa (4/4) vs v0 cmaa (4/4) is 4 - 4 = 0 is 1.9% CTK
v4 cmaa (8/8) vs v4 cmaa (8/8) is 8 - 8 = 0 is 1.9% CTK

Give the first unit -2 atk for being quite tired:

v0 cmaa (2/4) vs v0 cmaa (4/4) is 2 - 4 = -2 is 1.3% CTK
v4 cmaa (6/8) vs v4 cmaa (8/8) is 6 - 8 = -2 is 1.3% CTK