The last 30 kilometres weren't easy, with plenty of hills to navigate, but Cooper was far too strong for the chasing pack, remarkably riding away to claim a dominant win by two and a half minutes.
With Cooper coming into the stage 10 minutes down on the overall lead, the peloton wouldn't have been overly concerned with the chase, but to underline Cooper's strength, he was the only member of the breakaway to survive, with the peloton sweeping up all the dregs and sprinting for second place.
Cooper's victory is New Zealand's best of the year so far. Eight of the nine victories claimed by Kiwis this year were either at the National Championships, Oceania Championships or New Zealand Cycle Classic, while Patrick Bevin took out a team time trial with his BMC squad at the Tirreno Adriatico.
It is also the first individual stage win in an overseas tour since Cooper's success at the Tour of Hainan in November last year, and brings his tally of professional wins to eight - five of which have come since 2017.
The two-time national champion has also cut short a rough patch of form with the victory. Cooper had not finished in the top 10 of a stage all year - though he did claim the King of the Mountains at the Rhone-Alpes Isere Tour last month - and he celebrated in style when he crossed the line.
The tour ends this afternoon with a short stage five into Seoul.
* Yes, it's actually called the Tour de Korea. It's uproarious how many tours steal the Tour de France's "de" when they have no French ties at all.