Cooper has added Bay of Plenty's Mitch Karpik to the reserves after he showed his form for the development team last weekend.
"Marty McKenzie has earned a start at fullback and Mitchell Karpik comes on to the bench as loose forward cover. We've been hugely impressed with Mitch Karpik's work ethic and he had an outstanding game for the University of Waikato Chiefs Taua Development team last weekend. His performance has demanded a place in the 23 this week.
"This is only game four in what is a hugely demanding competition so we're focused on managing our players particularly Liam [Messam] and Tiaan [Falcon] who are both going extremely well but are carrying some niggles. Liam's leadership is critical to our group and we want to keep him fresh and hungry with some big derby matches in the coming weeks."
Cooper has made five changes to the starting 15 who battled to the impressive 41-28 win over the Bulls at home last Friday.
Canadian recruit Tyler Ardron (Bay of Plenty) comes in to the locking department to partner Brodie Retallick.
It's the Canadian's first start for the team, after an impressive cameo off the bench against the Blues in round three. Taleni Seu also comes in to the loose forwards after a rest last weekend.
In the backline, quick-footed Te Toiroa Tahuriorangi starts at halfback, after making a fantastic impact as a substitute against the Bulls last week.
Speedster Toni Pulu is back from injury for the first time this season at right wing and Marty McKenzie starts at fullback with brother Damian moving to first five-eighth.
It is a special evening for Waikato youngster Bailyn Sullivan who is in line to make his Investec Super Rugby debut off the bench in the number 23 jersey. Bay of Plenty's Sam Cane will play his 99th game for the Chiefs.
Chiefs v Sunwolves:
Aidan Ross (9), Nathan Harris (34), Angus Ta'avao (3), Brodie Retallick (86), Tyler Ardron (1), Lachlan Boshier (22), Sam Cane (98), Taleni Seu (30), Te Toiroa Tahuriorangi (3), Damian McKenzie (53), Solomon Alaimalo (12), Johnny Faauli (9), Anton Lienert-Brown (43), Toni Pulu (22), Marty McKenzie (9)
Reserves: Liam Polwart (9), Karl Tu'inukaufe (2), Jeff Thwaites (1), Michael Allardice (24), Mitchell Karpik (6), Brad Weber (45), Sean Wainui (3), Bailyn Sullivan (0)