I would first find (a - 2b^2)^2
a^2 - 4ab^2 + 4b^4
Then the problem becomes (a^2 - 4ab^2 + 4b^4)(a^2 - 4ab^2 + 4b^4)
Distribute the first over the second
a^4 - 4a^3b^2 +4ab^4 - 4a^3b^2 +16a^3b^4 - 16ab^6 +4a^2b^4 -16ab^6 + 16b^8
Simplify
a^4 - 8a^3b^2 +4ab^4 +16a^3b^4 - 32ab^6 +4a^2b^4 + 16b^8
rearrange
a^4 - 8a^3b^2 +16a^3b^4 + 4a^2b^4 + 4ab^4 - 32ab^6 + 16b^8
So the fourth term would be 4a^2b^4 (unless I made an error up there!)