"It's great for our championship," Cowper said. "We're really happy to be able to drop our 14th, obviously.
"People are saying we've got one hand on the cup."
Biggs came 12th in Masterton, while defending champion Hamish Auret of Whanganui had to withdraw from the event before round 5 with engine troubles.
Cowper no longer has the services of co-driver Lance Brasting, who broke his shoulder in an unrelated accident, and so has brought in near neighbour and "greenhorn" Andrew Bielby, who said he was "loving it".
To win another national title, Cowper just needs a steady and consistent round for the season finale at Egmont Village, Taranaki, on March 26.
"I'd like to think we're in the best position of anyone. There's a few guys up there," he said.
"If we were top three, we'd probably have it in the bag. Maybe as low as top five depending where those others are at the end of the day."
Competing on the Kiriwhakapapa Rd course at Mt Bruce on Sunday, Cowper and Bielby did not have it all their own way, as during the afternoon session they got stuck in the first hazard, picking up 80 penalty points.
"You'd think that would put us right back," Cowper said. "We tried really hard all afternoon to make our points back and we did. Ended up with the best score of the afternoon round."
Their best success was on the water hazard, as after choosing a different way to their rivals, they made it all the way through for no penalty points.
Windy City 4WD club hosted Round 5.