There are three ways to reach this mission from Area 4.
Option 1: Use the closest Gusher on the right side of the area and use the Inkrail to get on a tower. The tower is actually a pit underground. The kettle to Tumbling Splatforms is on the extra platforms, opposite side from the Inkrail and propeller.
Option 2: This is the same method for going to Propeller-Lift Fortress and Spinning Spreaders. Go underneath the area and use the left Inkrail. Use the Gusher and jump to the center platform leading to the boss kettle. There is an Inkrail leading to Octoling Uprising, but another Inkrail under the kettle, leading to the same tower to fall in.
Option 3: Similar to Option 2, after getting off the left Inkrail when fallen in the nearest pit of Area 4, do not use the Gusher. Perform a Squid Jump to the lone platform that holds a single Inkrail. Now, use a Splat Bomb, and throw it at the propeller. The propeller will head toward the platform you are currently on. The platform will take you to a platform with the Tumbling Splatforms kettle.
Head to the first Splatform, holding one Octotrooper, and defeat it. When off the Splatform, another one will be there, but guarded by two Octopods and one Octotrooper. The second Splatform does not hold any Octotroopers, but there is one waiting further along for you. Once they have been taken down, a launchpad awaits as well.
Two Octotroopers are guarding a cube Splatform. After getting past that, there are more cube Splatforms, which are guarded by an Octobomber.
A lone Octotrooper is on guard of two cube Splatforms, showing a rectangle. Getting past the Octotrooper is very simple. Throwing a Splat Bomb in its general area will splat it, leading you to the next Checkpoint.
The first Splatform on this checkpoint holds two Octotroopers on different sides of the platform. Getting past the two Splatforms will put you in a small area guarded by four Octopods. Once they are defeated, you will move on to the last checkpoint.
The two Splatforms are put in a way for you to perform a swimming squid jump, then swim up a wall. After that, there are three Splatforms above and the Zapfish is located on the top of the tower. Your only way of reaching the Zapfish is when the Splatforms are on its tallest point. If it is inked, you can swim up the Splatform to reach the Zapfish. Repeat the process two times to finally reach the Zapfish.