Right now, there is only 5 4-starred Saber : Nero, Artoria (alter), Siegfried, Chevalier d'eon, and Artoria (Lily).
Siegfried and Artoria (lily) is basically trash. Chevalier D'eon is a no-no because its very low attack. Nero is quite good. Finally, Artoria (alter) is the best saber because its high ATK and good NP.