Since 4e character classes work a bit differently than previous editions, why not start with this question:
What is a samurai's ROLE. I see suggestions of fighter, paladin, ranger, and warlord with all kinds of multiclassing options. But I think you really need to start with his role.
Is a samurai a noble-born battlefield general? If so, he is a warlord.
Is a samurai a whirling dervish of katana-full deathness? If so, he is a striker, most likely a ranger.
Is a samurai an honor-bound protector of his lord's lands and residents? If so, he is a defender...and I would personally start with a paladin instead of fighter and thematically replace all of the divine and radiant descriptions with honor challenges and ki shouts.
Also, correct me if I'm wrong, but isn't most of a samurai's armor made of toughened leather? Sure they have the metal "fish scale" armor over vital areas, but I thought all-metal armor was too restrictive and cumbersome for a samurai. I think they would be wearing the best light armor available in the game...in this case, it is called Hide in the PHB (but again, thematically, you can call it whatever you want as long as the stats are the same).