I've read this, hands of stone, SRL's autobiography, and Hitman: The Thomas Hearns story.
Four Kings was very good with the facts and figures that Kimball had clearly researched, it gave a good idea of the aftermath of each fight. I also liked the amateur backstory at the beginning. But ultimately it didn't really go into much detail about the fighters themselves. I thought SRL-Duran 1 & 2 got plenty of coverage in the book, I wouldn't say there was bias against Duran
Hands of Stone was a great read, and really did Duran's story and epic life justice. So much information about Duran's youth and life outside of the ring. All it was missing was narrative with Roberto's own perspective on how he felt about his big fights, and the great fighters he faced.
Sugar Ray Leonard's autobiography was my favourite, it was a really honest and personal account from Ray's own perspective. I loved reading about his preparation, including his thoughts and feelings leading up to his biggest fights. And the chaos of his life outside the ring
Hitman:The Thomas Hearns story was a good read, but it is more a narrative of his career, with newspaper quotes thrown in here and there. Doesn't really say much about the man himself.
I'd love to read a Marvin Hagler autobiography, but a guy like Marvin doesn't seem the type who'd be interested in writing one
All in all I'd recommend all of these books