Samurai Warriors 4 [戦国無双4] was a great game. Each faction got a nice story told about them, and I wasn't left unsatisfied with how each of them ended.
With Samurai Warriors 4-II, Koei has apparently decided that some factions and characters needed more fleshing out, to mixed results.
First: SW4-II adds one new character and five new battles over SW4. The new character, Naomasa Ii, has a flashy and fun moveset to play around with, and isn't a bad addition to the cast at all.
In terms of story content, I was mostly satisfied with what they did. Yoshitsugu's and Kojuro's campaigns, for example, left me liking the character a lot more than I did after SW4. Hisahide's campaign is great as a comic relief break between all of the more serious stories.
Noboyuki's campaign, however, is an inferior retread of SW4's Legend of the Sanada. Mitsunari's campaign also falls short, and seems redundant when Yoshitsugu's campaign treads most of the same ground, and does a better job at making you like Mitsunari than his own campaign.
The biggest problem with this game is that most of the campaigns put you in the same battles over and over, rarely getting a unique battle to themselves. The final two battles of SW4 are repeated time and time again in nearly every campaign here. Each campaign's lead character fights for different reasons, sure, but it gets really old fighting the same enemies on the same map over and over.
The way they ordered the campaigns on the menu was really poor. For example, Hisahide's and Koshosho's campaigns are the only ones with the Honnoji map, but those two campaigns are listed one right after another. If they were more spread out, returning to that map wouldn't be bad at all, but as it sits, they're two battles apart if you play through in order.
In the end, SW4 was a better game, and should be played before/instead of this one. The campaigns were far less redundant, and told more of the story. It had a better side mode, and I'm more likely to come back to it later than I am this game.
If you enjoyed SW4, I'd only recommend getting SW4-II if you can find it for $20 or less. Even that's a stretch.
If you want to play as Naomasa,
Samurai Warriors 4: Empires [戦国無双4 Empires] and
Warriors Orochi 4 [無双 OROCHI 3] are far better options.