dark souls- Four Kings

It depends on whether it's NG or NG+ (and beyond). In NG, load up on 20 estus, put on Havel's armor, eat some green blossom, and tank it. Just drink when you need to and 2-hand your weapon and swing. If you have a weapon buff, use it (lightning and magic
both work well), and also great combustion can eat into their health pretty quickly. Also, as they are dying you can get some more hits in to do extra damage, which saves you from having to deal more damage later in the fight.

An alternative solution is to be a fast character with a fast weapon that you can swing rapidly. Go into the fight in red tearstone range and dodge attacks while getting hits in as often as you can, and make sure you use green blossom. It's more difficult
to fight this way, but can be a better approach if you have a character with low health.

I've found the best course of action is to get a lightning uchigatana+5, red tearstone ring, and green blossom. Their attacks are pretty easy to dodge, so just get in there and start slicing.

Make sure to continue attacking a King during his death animation. The extra damage you do counts toward their total hp and can shorten the fight considerably.

@Judas

"9. Dark Souls 1 was practically the same way as well. You collect the "four lord souls" to kill Gwyn and light the bonfire. Dark Souls 2 you collect the "four great souls" to kill Nashandra and become king."

The king of what exactly? It's not like whatevertheplacewascalledinDaS2 has much for you to rule over. I mean, you killed the bosses so what's left is a handful of NPCs and a bunch of rotting undead. Wow, it's so great to be a king of that place, eh? At
least in DaS1 you either gave the cursed (and the uncursed as well) more time to figure things out by strengthening the first flame or you decided it's time for the true Darkness to come. Those things both changed the world, the latter option changes it big
time. But becoming a king in DaS2? Changes literally nothing. You killed Nashandra, big deal. Nothing changed. You didn't cure the curse or even yourself. The place won't be thriving after you become a king since it's just full of cursed or undead things.
So you're the king of nothing. I stand behind my words: in DaS2 you only kill bosses for the sake of killing them while in DaS1 there was still some kind of a plot, an actual reason to your actions.

Man... I spent well over 2000 hours in the original Dark Souls. I slogged through Blighttown, I cursed the Four Kings, and i HATED HATED HATED the crystal cave but I did it time and time again to unlock the achievement im proudest of: The Dark Soul. That
was completely worth it in the end.

Also Mile High Club. Beating that level on Veteran is the reason imma die young.

got a level 89 currently going through the Four Kings.

and a level 17 trying to figure out what I should do next.

There is a Dark souls Wikia that is pretty good in explaining certain things like Invasion SL range and coop SL range