Samurai Warriors 4-II is a stand-alond sequel to 4 as close to the old Xtreme Legends titles as allowed. It has characters and modes not available in the original SW4, but does not have all of 4's content.
The Sanada game follows the Sanada clan specifically in a story in which time progresses and you see Yukimura and Nobuyuki grow up. All of the battles and content is geared toward that story, but the rest of the SW cast is available for non-canon battling.
Thanks, is there one you would recommend more? I have no attachment to the story nor the Sanada clan, I'm more of a bit of a Three Kingdom person honestly but looking for a bit of a mix up I guess. I think the last musou game I really enjoyed was probably Fire Emblem. It just had the right mix of letting me beat up mobs while occasionally needing to think about where to throw sword in the map to make sure I don't fail objectives.