You didn't destroyed the fourth Hind yourself since it crash on the ground !
The same thing will happen if you ask one of your team member to destroy something, you will not get credit for that !
Something similar happen to me in the mission where you have to blow three shilka.
I place a statchel charge next to the first and blow it, i approach the second shilka (the one on the road) i killed the crew members next to it and take the gunner position, about ten second later a Hind was coming at me, so i start shooting but the Hind got me first and he destroy the shilka, the third one was destroyed by one of my team member. Finally i got credit for only one kill, because the second shilka was destroyed by the Hind !
You get credit for what you really did yourself !!
