Okay, the user wants me to translate "four years" into Simplified Chinese. Let me check the rules again. The first rule says to translate all content into Simplified Chinese. The second rule states that if the content is already in Simplified Chinese, output it unchanged. The third rule is to provide only the translation result without explanations.
So, "four years" in English needs to be translated. The direct translation would be "四年". Since "四年" is already in Simplified Chinese, according to rule 2, I should output it unchanged. The user also emphasized that they want only the translation result, no explanations. Therefore, the correct response is just "四年".