[Number4]

5 SP are standard, because after a normal outing to the best of his capabilities, a pitcher needs 4 rest days.
You could go by with 4 SP and 3 rest days, but you would have to cut the pitchers outings short or they'll start their next games tired.
This means that even if your pitcher is going strong, you'll have to pull him and give the game to a middle reliever - while a pitcher pitching as hard as he can, will on a good outing often last until the setup or closer.

However, it cuts out the #5 SP, which is usually the worst one. I'd say the baseline for SPs with 4 rest days are 100 pitches, with 3 80, so in terms of innings more or less the same amount. But your bullpen will be more tired with the 4 SPs - the 5th starter throws way more pitches than your worst reliever.
I'd say only use 4 starters if you are confident in your bullpen in quality and stamina. Or if you add an extra reliever, sacrificing a position player. Especially DH teams don't need more than 3 bench players, every more is an luxus - backup catcher, utility infielder and 4th outfielder.

6 SP is nonsense to me - your starters are not going to be more rested than with 5 starters, except after a rare mammoth effort, and you're just giving innings from the #1 to #5 starters to the #6 starter, which will be worse.