Your weapon is +4 or is that a typo? If not, it should be +15 and hopefully buffed since you're a magic user - Crystal Magic Weapon only needs something like 25 INT to use.
The standard advice for 4 Kings is just hack away up close with high poise and heal when necessary - Put on something like Havel, Giants or Stone armour sets and just tank it, two-handing your weapon.
Since you're also a magic user, you can also use your most powerful/fastest spells. Use crystal homing mass/spear etc - put on the bellowing ring, but maybe not dusk crown since it also increases your damage taken from magic as well as buffing your own magic. On that note, using the crest shield with it's 80% magic reduction is also recommended.
Any stamina regen items or Green blossom is useful - if you were going for the bellowing ring but not dusk crown, you are free to use mask of Child and cloranthy ring.
Lastly remember to keep hacking away even when one king has died, the last couple of seconds as they disappear you can still inflict damage (same goes for Pinwheel when he warps, not sure if there are any other examples)
I was grinning like a fool watching this. So many memories from my first time playing, so jealous! Keep recording your playthrough