Four, then five.
That's sufficient.

Not with a cliffhanger like
ESB's! Granted,
ROTJ may be the second-worst stand-alone film in the Saga, but it's
still mandatory.
By the way, if anyone wants to know how I rank the individual films in the
Star Wars Saga, from best to worst, here goes:
1.
Episode III: Revenge Of The Sith (2005)
2.
Episode V: The Empire Strikes Back (1980)
3.
Episode II: Attack Of The Clones (2002)
4.
Episode IV: A New Hope (1977)
5.
Episode VI: Return Of The Jedi (1983)
6.
Episode I: The Phantom Menace (1999)
Okay, okay, I'm sure there'll be those who'll think it positively
outrageous 
that I would rank
any film in the Prequel Trilogy above
ESB or even
ANH, but truth be told, the Original Trilogy's become somewhat...
overfamiliar to me and
ROTS still feels somewhat new and fresh. Also, I think
AOTC is
extremely underrated. (And for crying out loud, just get over that "I don't like sand" dialogue, you whiny fanboys!

) And just because I rank
ROTJ and
TPM last, doesn't mean that I think they're necessarily
bad, it's just that I think they're the least effective as stand-alone entities.