I voted for 4. For me it's the best option with }{ on a separate line.

    html {
    }{
      & body {
        width: 100%;
      }
    }

You have a clear distinction between the "standar" rules for the current object and then a different set of rules for nested ones. While reading css, on the other options you can mix both styles which (specially with the optional & ) can become easy to misunderstand.