Minimum rakats in salah tahajjud is two. Maximum is unlimited, but should be performed in two rakats, one salam, like in salah subh. So if you want perform salah tahajjud in four rakats, you should perform two rakats, finished with salam, then start again from beginning. You can recite any surah.
Its optimal recommended amount is 8 rakats, because this was the general practice of the Messenger of Allah (blessings and peace be upon him, his family, and companions). [Hindiyya, quoting Fath al-Qadir] meaning you will pray 2 rakats four times