根据我的理解翻译了四个同义句子，求问有没语法问题，其中哪种使用方法更容易见到更地道，哪种是根本不会有人用的非常荒谬的呢。或者还有哪些翻译是更适合的？
1 He is not so foolish as not to understand it.
2 He is not too foolish to understand it.
3 He is not foolish enough not to understand it.
4 He is not so foolish that he can't understand it.
【1】 He is not so foolish as not to understand it.
【2】He is not too foolish to understand it.
【3】He is not foolish enough not to understand it.
【4】 He is not so foolish that he can't understand it.
以上都是OK的。我再提供以下2个供网友参考。
His stupidity won’t go so far as to let him fail to understand it.
He will not be so foolish as to be unable to understand it.
