The Four Kings were once great leaders of New Londo, and, in recognition of their foresight, Gwyn granted them their ranks and apportioned fragments of a Lord Soul discovered from the First Flame. When the Four Kings were seduced by evil, their knights became Darkwraiths, users of the art of Lifedrain. In an effort to contain the threat they posed, New Londo was flooded, trapping the Four Kings and the Darkwraiths in the Abyss, and sacrificing the city of New Londo and the countless lives therein.
Summon signs are most commonly found at the beginning of the New Londo Ruins, before crossing the bridges. Some can also be found right before the boss door.
Witch Beatrice's summon sign can be found hidden around a corner just before crossing the short bridge to the Fog Door, but only if you have already summoned her for the Moonlight Butterfly fight. It may prove more than useful if you are a light build.
Unequipping the Covenant of Artorias will result in the player dying immediately; you cannot unequip this ring while fighting the Four Kings. After the battle has been completed, the ring is no longer necessary and may be unequipped without dying.
Upon entry, the player will have time to cast any sort of buffs, during which a single king will appear; the second will join approximately one minute later; the last two appearing in the minute following. It is vital to eliminate each King quickly to avoid being surrounded.
There is no obligation to kill any specific amount of kings, so it is possible to spread out the damage to several of them. This can be done by dealing massive postmortem damage to a single King, as they can still be hurt during their death animation.
Power Within adds decent amounts of damage to attacks and can help destroy individual Kings quickly before the next one appears.
The damage dealt by the Kings in NG+ is far greater than in NG, so it may prove difficult to absorb their attacks with heavy armor.
Staying close to a King will force the lower portion of the blade to hit, minimizing damage. This is true even on NG+, as getting hit by the upper section of their blade will deal heavy damage. This will also prevent it from using some magic attacks (namely, the homing shot) and the unengaged Kings tend to use melee at most distances.
Using a ranged attack and staying at around 1-1.5 roll distance away in front of the king (just within the reach of his sword) makes him rely mostly on the vertical slashes and the occasional Homing Abyss Arrow or Life Drain. The Life Drain is also easier avoided since it looks less like Wrath of the Kings at this distance then it does close up, and the player already has a head start on getting out of reach.
Their homing attacks can be deflected by using Force or Wrath of the Gods, although it is safer to avoid the attacks entirely. Due to the speed at which the King approaches, the player will become vulnerable to other attacks during the casting animation. Deflecting the large homing shot will likely be fatal, since the duration of the attack lasts a fair while and doesn't collide with anything other than the player.
Initially, it can be hard to determine how close a king is; they can appear close, when they are in fact out of reach. It's a good idea to run into a king before attacking, not only to be sure your attacks will land, but also because being close to a king can cause his attacks to miss.
Using armor with good poise (Havel's Set will be the best for this as it has good defense without using materials) while staying close to the kings will allow you to tank through the hits (as they deal significantly less damage while you are next to them) and constantly hit them. Be wary, though, as full heavy armor is extremely heavy so fat rolling or even no rolling is likely. Using Green Blossoms and the Grass Crest Shield is also recommended.
The battle arena is pitch-black with no visible features. While there are invisible walls, rarely do they create complications.
The Kings spawn in one after another at a rate of about 1 every 40 seconds, and all four can be present at once. If one is killed before 40 seconds elapses, another will take its place, immediately resetting the spawn timer. Kings will continue to spawn for as long as the battle lasts, unless there are already four Kings present.
Although up to four may be present during the fight, only one will engage the player in direct combat. The other Kings, however, are able to attack with their sword if the player is close enough, or cast magic homing missiles when the player is too far.
Each of the Kings' health is equal to a bit less than ¼ of the total bar. This means that after killing four individual Kings, the boss itself may not yet be defeated and a fifth King will spawn. This can be avoided by attacking the Kings as they are dying, which can do additional damage to the life bar.
Bonus damage dealt due to a weapon's divine effect applies to the individual King's health, but not the collective health bar. This means that divine weapons will kill the individual Kings faster, but the fight itself will take just as long as if the player had a non-divine weapon with the same attack rating.
It is possible to bypass most of the ruins and reach the Kings by going from the bonfire between Darkroot Basin and the Valley of Drakes, cross the bridge in the Valley of Drakes enter New Londo. From there one can take the elevator on the left that leads up to the seal. From here it is possible to drop down right next to the fog gate.
Two wide sweeping slashes with the King's sword. May occasionally stop after one attack. These have excellent tracking and are quite damaging to health should a critical hit land. Stamina is largely drained when blocking both consecutively. They can be rolled under, and can be sidestepped away from without taking damage.
Single powerful straight thrust with the Four King's Sword. It has the longest reach of all melee attacks and deals massive damage when absorbed head on. This attack is only triggered when the player is at a distance and cannot be strafed; although it has poor tracking and can be dodged.
Extremely powerful, large area-of-effect attack in which the King will spin around, covering himself with his left "wing" and emanate a purple aura. Blocking is most successful when using a high stability shield with good magic reduction. Otherwise, players should release their lock and run away when the King begins his attack.
The King will fire off six energy shots that home in on the player. These are much quicker than Homing Abyss Arrow, but they also have much weaker homing capabilities, allowing the player to easily roll or move away when they approach. Getting hit by multiple shots will deal immense damage.
The King will fire off a slow moving, large, purple projectile that has high tracking capability and powerful magic damage. It cannot be evaded and may only be blocked by a high stability shield with high magic resistance; which will likely drain the entire stamina bar, but may also reduce the damage. The only way to avoid this attack is to outrun it until the projectile disappears. A new king might spawn within that time period.
This is a Humanity stealing grab attack, and, like all grab attacks, it is unblockable. Its animation is similar to Wrath of the Kings: the King will spin around and pause, holding its wing/arm in preparation while emanating a colorless aura. If the player is slow to react, his forward grabbing dash will be inescapable. Counter-intuitively, it can be avoided by moving closer to the attacking king before they begin their lunge. In extreme proximity the attack will simply go over the player's head. This attack can be fatal if the player is not using armor with high magic defense. Unlike other grab attacks, the King can still be damaged by aiding phantoms as a player is drained; however, this will only deal damage to the Four Kings' collective health bar rather than the individual King.
These stats are sourced from FuturePress's Official Dark Souls Strategy Guide.