After countless hours I still haven't beaten NG+ Four Kings...
That fight is a lot harder than in normal game..
Each king has 4k+ hp and they hit like trucks and they seem to spawn faster than in normal game...
I tried summoning but it increase the Hp of the Kings and doesn't make the fight easier...
I wasted 2 hours leveling to be able to use the true great sword of Astoria to realize in the end it was not doing better damage than my current Occult Great Scythe +5 too ;( ( farmed 20 levels for nothing cry).
I have finally found a strategy that seem to work though.
It's going to be Heavy Poise gear + iron flesh + crystal ring shield spam...
Last try using that strat I got them to 25% health...
Only problem is you have 3 cast of iron flesh and 4 kings... ( and going melee you can barely kill 1 king within 1 cast of iron flesh, hoping it will be faster with the crystal ring shield)