What are trying to say? Increasing the cost does what?, you said it yourself, the 8 will most likely beat the 5. So you want to handicap this unit more by making it more expensive? And you want to restrict it even more by limiting build limits?
I like each unit to have some unique ability in AA. To overcome the math of 8 beats 5, you give some flavor to the new unit, but to balance in this case i need to raise the price +1, If you keep it at 4-4-2-8 and include the “roll 1 choose loss” thing its a bit in the other direction. If you feel its now balanced than make your case. The other thing is 4 is the crossing of the threshold for AA land units because this is D6 system and 4 out of 6 can be exploited by buying about 6 of them and surrounding them with infantry.
Wait: i have a new idea! Heavy tanks boost the attack of Infantry +1…like artillery, because the gun is a larger caliber. THis is a good rule and i modify it to that:
New rules:
4-4-2-8, and boost 1 matching infantry in attack +1…ok?