|Game: Sonic Adventure / Sonic Adventure DX|
|Level: Mystic Ruins|
|Hits to defeat: 5|
|Fought by: Sonic, Tails, Knuckles|
|Next form: Chaos 6|
Chaos 4 is Sonic's third, and Tails' or Knuckles's second boss in Sonic Adventure. Sonic and Tails have been scrambling to collect the Chaos Emeralds before Robotnik finds them and feeds them to Chaos; their search takes them to Ice Cap, where Sonic finds the green gem - this being the fourth Emerald now accounted for.
Strolling back across Mystic Ruins, the friends run into Knuckles, making his first appearance in Sonic's story. The echidna is unusually belligerent, even for him, and demands that they hand over their Emeralds - he has been tricked by Robotnik into believing Sonic is collecting Master Emerald pieces. Sonic's refusal leads to a pre-boss skirmish between Sonic and Knuckles, where the echidna must be conked on the head via homing attack three times.
During the scuffle, Sonic's two emeralds (the green one from Ice Cap and the blue one from Windy Valley) are knocked out of his grasp, at which point the Egg-O-Matic makes its appearance. Robotnik grabs the jewels and, laughing at Knuckles, tosses them to Chaos 2, who transforms straight up to Chaos 4. In this form he has a vaguely shark-like appearance, and the player character steps in to fight him in a lilly-filled pond.
As in previous incarnations, Chaos 4 steps up his attacks depending on how damaged he is. At full health, he will simply swim around sinking Sonic's lilly-pads and occasionally throwing out an energy wave. He can be hit in the brain whenever he surfaces. After one hit, the beast takes to dividing himself up into four bouncing orbs which pursue Sonic; on two hits, he starts throwing extendo-punches with his watery fins. At three hits, Chaos starts sending out high-speed waves on top of all his other techniques. Five hits finishes the boss off.
Chaos departs by melting away into the water table, while Eggman flees up to his hovering Egg Carrier. Not willing to let him get away that easily, Sonic and Tails dash up to Tails' Workshop, hopping into the Tornado for Sky Chase Act 1.