Written and illustrated by Eiichiro Oda, One Piece has been going strong for over 25 years and is one of the most popular pieces of fiction. Being a long-running manga with little to no breaks, One Piece has a huge number of sagas, with the Four Emperors Saga being the latest one.
The Four Emperors saga has proven to be the longest in the story, starting with Zou and recently ending with the conclusion of the Wano Country arc. The conclusion of this saga marks the beginning of the final, still unknown, saga. The Four Emperors Saga, in particular, holds no bad arcs from start to finish, with each of them being incredibly well-written.
4 Zou Arc
The Zou arc takes place after the Dressrosa arc following the fall of Donquixote Doflamingo in Dressrosa. It also paves way for the Whole Cake Island arc with Sanji's departure from the crew. Zou is the 28th arc in the series and the first arc to kick off the Yonko Saga. It spans from chapter 802 to 824, making it quite a short arc when compared to some others in the saga.
After Doflamingo's defeat, Luffy and the Straw Hats that fought on Dressrosa, including the likes of Zoro, arrive at Zou, the giant moving elephant island that traverses the seas perpetually. This island is revealed to be the home to a special race called the Minks. The Straw Hats head to Zou to catch up with the other crew members, namely Sanji, Nami, Brook, and Chopper. However, when the crew arrives there, news of Sanji's departure is broken to them. It is revealed that the latter was forced into an arranged marriage by threatening his adoptive father, Zeff's life. Zou sees the crew meet a huge number of Minks who are shown to be very welcoming. Throughout the arc, fans learn of the tragedy that occurred on the island when Jack, one of Kaido's underlings, attacks them in search of Raizo, one of Oden's vassals. Raizo is later revealed to be safe and the crew discover 1 out of 4 Road Poneglyphs that lead towards Laugh Tale.
3 Reverie Arc
The Reverie arc is a short and information-heavy arc that takes place after Whole Cake Island, and before the start of Wano. It's the 30th arc in One Piece and consists only of 6 chapters, from chapters 903 to 908, which would be episodes 878 to 889 in the anime. The Reverie is an important event that, at this point, had been foreshadowed for quite a while in the story. It sees all the world's royalty affiliated with the World Government meet up to discuss world issues. In this arc, the Revolutionary Army reveal their plans to take action to free their comrade Bartholomew Kuma, who has been made a slave by the Celestial Dragons. At the same time, their main goal is to declare war on the Celestial Dragons. Most importantly, the Reverie arc introduces fans to Imu, a mysterious individual sitting upon the Empty Throne, and seemingly the ruler of the whole world, who orders for a light to be extinguished.
Reverie sees the return of countless old and beloved characters, most importantly Nefertari Vivi, the honorary Straw Hat that was left back in Alabasta. Alongside her, other princesses make their return, namely Rebecca from Dressrosa, and Shirahoshi from Fishman Island. Other characters such as Rob Lucci and Kaku, now CP0 members, also make their appearance after quite a while.
2 Whole Cake Island
The Whole Cake Island arc is the first arc where the crew come face to face with a Yonko and their whole crew. Being the 29th arc in the series, it comes right after the Zou arc and falls just before the short arc, Reverie. Whole Cake Island serves as a huge set-up for the saga's finale, Wano. It is a fairly long arc, being made up of 78 chapters, from chapter 825 until 902, which would be episodes 783 to 877 in the anime. After the split of the Straw Hat crew on Zou, Luffy's group's main mission is to rescue Sanji and escape from Big Mom's territory, which is easier said than done. Whole Cake Island revolves heavily around Sanji and reveals his true family is the Vinsmoke Family, which makes up the Germa 66, the evil kingdom known to everyone in North Blue. Sanji's early life is revealed to have been riddled with abuse from his father and brothers, where only his mother and older sister Reiju show him kindness.
The arc also heavily focuses on the Charlotte family that makes up a sizable part of the Big Mom Pirates. Most importantly, Charlotte Linlin, the leader, and her right-hand and second son, Katakuri. The arc sees Luffy acquire a Road Poneglyph and, while escaping get stronger by fighting Katakuri and defeating him, being declared the Fifth Emperor of the Sea in the process.
1 Wano Arc
The Wano arc is the culmination of every major event after the time skip of One Piece. Indeed, the Wano arc wasn't just the conclusion of the Yonko Saga, but every single arc after the time skip, which makes it doubly special. Wano is officially the 31st and longest arc in One Piece, starting with chapter 909 and finishing recently with chapter 1057. It sees Luffy and the crew deal with not one but two Emperors of the Sea and their crews. Reaching the day of the Onigashima Raid required patience and caution on their part, but with luck on their side, the alliance successfully managed to launch the offensive on the Emperors. The goal of the raid was to open the borders of Wano Country and liberate the land from the clutches of Kaido and Orochi, just like the late Kozuki Oden wished for. The arc reaches its conclusion after Luffy unlocks Gear 5th and uses the full potential of his Devil Fruit, revealed to be the Mythical Zoan Hito Hito no Mi, Model: Nika, to defeat the Yonko, Kaido. Meanwhile, Kid and Law handled Big Mom.
Other notable characters introduced in this arc were the Tobi Roppo, a group of fighters under Kaido. Fans were also introduced to Kaido's child, Yamato, who turned out to be a powerhouse themsleve, being able to hold their father down while Luffy recovered from his first second defeat to the Yonko. The ending of Wano was very emotional and saw the Straw Hats wish farewell to their Wano friends, sailing towards the One Piece, now closer than ever before.