At 12% ABV, a serving size is 5oz.[0] There's 23.5oz in a FourLoco.
IMPORTANT EDIT: Apparently these labeling changes were made specifically due to a lawsuit against FourLoco.[1] So you're totally right, a serving was 1 can, despite having almost 5 drinks worth of booze.
https://www.fatsecret.com/calories-nutrition/four-loko/four-...