Cooper performed well in the opening moto and passed championship leader Matt Moss on his way to a second-place finish behind Monea.
"I started tight in the first race then settled in," Cooper said.
"I found my rhythm and worked my way past Matt Moss and into second."
Disaster struck at the start of the second moto when a collision with Jake Moss saw Cooper come down hard on his knee and unable to finish the race.
He was waiting for a medical assessment of the extent of the injury before this weekend's fifth round near Perth.
New Zealand MX2 champion Kayne Lamont (Mangakino) continued as a front-runner in the Pro Lites (250cc) category.
Husqvarna rider Lamont led from start to finish in the opening moto at Murray Bridge but was 11th in the second race after losing ground with a first-lap spill.
That earned Lamont third overall for the weekend and he remains second in the Pro Lites title chase trailing Yamaha's Luke Clout by 12 points. Lamont enjoys a seven point advantage over fellow Kiwi Hamish Harwood (Takaka) who rode his KTM to a ninth and a fourth at Murray Bridge.