2Sum has an AC rate higher than 20 percent.
If a Leetcode problem is at the easy level, then its AC rate is lower than 20 percent.  All Leetcode problems that are recommended to novices are easy.  A Leetode problem is either easy or hard. Leetcode problems that are starred by more than 1 thousand users are hard. 2Sum is recommended to novices.  4Sum is starred by more than 1 thousand users.
∀x (Easy(x) → LowerAC(x)) ∀x (Recommended(x) → Easy(x)) ∀x (Easy(x) ⊕ Hard(x)) ∀x (Starred(x) → Hard(x)) Recommended(twosum)  Starred(foursum)