3) “True” form, End of the First Age - Second Age,until the downfall of Numenor. After battle on Tol-in-Gaurhoth Sauron was not in favor, and after War of Wrath he lost his master Melkor. He refused to return to Valinor with Eonwe and stayed in Middle Earth. For 600 years he begin to slowly lose his mind. His hair became gray and his beauty fades away.
4) End of the Second Age. Sauron dies during downfall of Numenor. His spirit can’t take a “fair” form anymore - since this moment his physical form became horrible