1. How kind he is!

2. How kind he isn't!
3. What a nice boy he is!
4. What a nice boy he isn't!

(#1 is correct. However, what about #2, the negative form? Can we use this way? And #3 and #4 as well?)

This expression is not used for the negatives. 1 and 3 are perfect, but you should rephrase 2 and 4 like this:

He isn't very kind, is he?

Yes, you can use positive and negative forms of sentences with expressions of admiration or astonishment like "How kind" and "What a nice boy." Here's how each of the sentences can be understood:

1. "How kind he is!" - This sentence is expressing admiration for the person's kindness. It is a positive statement.

2. "How kind he isn't!" - This sentence is expressing astonishment or sarcasm about the person's lack of kindness. It is a negative statement. While it may not be as common as the positive form, it is grammatically correct and can be used to convey a specific tone or emphasis.

3. "What a nice boy he is!" - This sentence is expressing admiration for the person, emphasizing their niceness. It is a positive statement.

4. "What a nice boy he isn't!" - This sentence is expressing astonishment or criticism, emphasizing that the person is not actually nice. It is a negative statement. Similar to the negative form in #2, it may not be as commonly used, but it is grammatically correct and can be used to convey a specific tone or emphasis.

In summary, while the positive forms (#1 and #3) are more common, you can use the negative forms (#2 and #4) to express astonishment, sarcasm, or emphasis on the opposite characteristic.