Yeah, I'm a decent amount of hours into OPW4 now and it's a pretty solid musou.
I like that upgrading the general character map increases stats for every playable character, so you don't have to upgrade everyone from scratch. The sub-missions are fun and the speed of the game is great.
Main story mode is a decent best of moments of a few arcs. The original story part is just for the final Wano arc since the manga arc was still running.
Character unlocks aren't too bad, some of the characters are really distinct and fun to play as. Especially some of the sky type characters that just air dash and air combo non-stop for half the map. Some characters seem pretty shitty tier though. I feel like this is a musou that would benefit from character swapping. Maybe in OPM5.
Overall worth $13. Might pick up the new DLC pack but $30 for like 9 characters is steep, and by the time packs 2/3 come out I'll probably be done with this and moved on, so eh, doubt I'll pick it up.