Was it Grevious who had 4 lightsabers ? Does that make it better than one ?
In his case, yes. He wasn't a force user, so he had a big disadvantage compared to a lightsaber-wielding Jedi or Sith, but he made up for that by being mostly cyborg and having 4 lightsabers. If you're a jedi you're not really used to fighting against another lightsaber-wielder in a war (because there were only two Sith), although you have done so in training of course. But who can prepare for a fight with a cyborg wielding 4 lightsabers and swinging them around in every direction constantly? Very few people. That's why Grievous killed so many Jedi.
Plus, in EPIII, why did Palpatine have a lightsaber at all if he could do the whole force lightning thing?
Well, if he'd been attacked with blasters, he needs some way to defend himself, right? He could just dodged everything, but having a lightsaber means having the ability to go full melee while dodging and reflecting blasts.
Moreover, you can't win a fight against another Jedi/Sith without a lightsaber if that other person has one. They could just use their lightsaber to block your lightning.