ONE PIECE: PIRATE WARRIORS 4

Once Dramatic Log is over, go into the pre-battle screen and take a look at what crew members are closest to reaching this. Many of the characters in this list are actually playable, meaning you will always get experience for that character; some others are chance encounters depending on the map, story, or random generators; the last might come from playing online with characters that are more or less commonly used (I played with so many Kaido mains it was ridiculous).

If you have the majority of the Straw Hat gang at 4, play a few episodes with them to get some experience with them. If you need some of the lesser known crew members, perhaps finding a specific Treasure Log where they're more likely to appear would be a better decision. Work on it slowly but surely and make sure you use Brothers' Sake Cups to give you added experience; trust me, it will help.