I'm confused about collecting the weapons parts in 3

As of the beginning of mission 7, the airfield, 4 weapon parts remain for me to get out of 15,, so I have 11 parts now. But there's 20 more upgraded parts needed for the base game weapons, huh? I don't get how only 4 parts can grant me 20 more upgrades. Granted I don't have any stock upgrades, so that's 8 upgrades right there, assuming one part give me one stock upgrade for all 4 base game weapons. How can 4 remaining weapons parts grant me 12 more upgrades ?