If you're looking for manga similar to One Piece Special 4: The Detective Memoirs of Chief Straw Hat Luffy, you might like these titles.
The world's greatest exposition of the pirates, by the pirates, for the pirates—the Pirates Festival. Luffy and the rest of the Straw Hat Crew receive an invitation from its host Buena Festa who is known as the Master of Festivities. They arrive to find a venue packed with glamorous pavilions and many pirates including the ones from the Worst Generation. The place is electric.
Who says pirates can only be cutthroats and bloodthirsty pagans? Meet Kid, the world's most notorious pirate and womanizer, and his rowdy crew that consists of not only pirates but also a priest and a mermaid?!
Long, long ago in a certain land, the world was disturbed by a "demon". The demon was "deformed" and didn't even stand 6 feet tall. His jet-black eyes sank into the depths of hell. The people seeing the atrocious and blood-starved appearance of this "deformed demon", trembled in fear. In the tenth year of the Kan'ei era (1633), during the early years of the Edo period, one such demon still exists...as to why "deformed", the reason will astonish you!
This is the story of Coco, an orphan kid who wanted to be a pirate, and his role model, "Crazy" Barts, a pirate without a port who is feared by the peasants but despised by the other pirates.
A princess who holds a piece of a legendary treasure map known as the "Stone of the Sea" willingly joins a famous pirate in order to avoid becoming the twelveth wife of some 72 year old king.
What happens when you combine pirates with some sort of Battle Royale? You get Dead Flag. An entire class has been selected to participate in it. They have ten days to rack up the most points by plundering, looting, and murdering. The top thirteen individuals win Dead Flag.