"We're all very proud - to be honest, I'm having a lot more fun now watching the kids than when I was riding myself," he said.
All four children began mountain biking at a young age, heading out for casual rides with their dad around Rotorua's trails.
The first big success came in 2012 when Amber, now 19, placed third at the junior World Cup at Mont-Sainte-Anne, Canada.
At the weekend she topped the field in her age group, and also managed a podium finish in the Elite Women's race, finishing third.
Elsewhere, 17-year-old Nathan was a hair's breadth away from taking gold in the Under-19 boys before being pipped to the post by Tauranga's Harrison Mudgway in a dramatic photo finish.
However, at least there was some family company up on the podium - cousin Daniel, 17, was there to pick up his bronze medal.
Fifteen year-old Taylor's third-place finish in the Under 17 boys' completed the Johnston medal haul.
Some parents might worry about their kids flying down steep mountain bike tracks, but Carl Johnston said he was more concerned about the traffic they encountered on the way to practice.
"You do worry sometimes, but I have great faith in their skills. I'm more worried about them cycling on the roads than I am about them on the tracks at the Redwoods."
It's not hard to see where this faith comes from - all four are out on the trails almost every day.
"They get home from school, get on their bikes and head out, and we see them at teatime," he said.
After the successful weekend, preparations have already begun for a trip to Toowoomba, Queensland, where the four Johnstons will be riding at the 2015 Oceania Championships, which begins February 25.
It will be the first time competing outside New Zealand for the three boys, while Amber will again be riding as a member of the NZXC cross-country team.
With a household full of talented mountain bikers, is there ever any competition between the four?
"Only every day!" Carl said.
"They're riding together all the time, so they use each other as training partners."
For the Johnston family, sibling rivalry is definitely a good thing.