FINALLY got to spend a little time with Sengoku Musou 4 after several days of packing/moving/unpacking. I was wrong about triangle attacks being completely replaced by the dash attacks-- what they've done is make it so that you get two different attack points to start from, each with its own branching chain attack paths (so regular attacks/combos starting with square, then dashing attacks starting with triangle).
The game allows you two pick characters and swap between them at any time. The difference between this and Musou Orochi is that your partner is on the battlefield with you and will be off doing his/her own thing, similar to the Chronicles games on the 3DS. You can stop the game play at any time and pull up a mini map that allows you to command your partner to target a specific point/general on the map.
Sengoku Musou 4 is hands-down the best-looking PS3 musou game yet. It's bright, colorful, and has the most detailed character models and stages yet. The framerate stutters here and there, but is otherwise pretty stable on both the PS3 and Vita. Both games offer the now-stand cross-save option, and now include an option to auto-upload saves to the cloud.
I've only tried the main story mode so far and picked Takeda Shingen's path. You get six characters to select from the start (went with Sanada Yukimura's big brother, Noboyuki, with Kunoichi as back up) and four stages for this scenario were shown. Can't wait to play more!