The 26-year-old's Korean Grand Prix victory, his eighth in 14 races, left him potentially one week away from clinching the title - the next race is in Japan on Sunday - but Horner still preferred to talk of if rather than when.
As far as the sport is concerned, the title is done and dusted however.
Only two other drivers - Germany's Michael Schumacher and the late Argentine great Juan Manuel Fangio - have ever won four successive titles and Horner said people would have to put Vettel on a par with them.
"Should he achieve it (winning four titles in a row), I think they will have to," he told reporters. "He has got to be right up there. He has got to be," added the Briton.
"The kind of level that he is performing at is unbelievable in many respects...what has been really impressive is that he has continued to develop, continued to grow as he has gained more experience.
"To have won the ratio of races that he has is quite remarkable."
Having claimed a clutch of records including the youngest race winner, youngest champion and triple champion, the German would also be the youngest ever quadruple champion.
Vettel has 34 race wins to his credit, from 115 starts, and has dominated the last four, from Belgium to Italy, Singapore and now the southern tip of South Korea.
He won in Japan last year and rates it as his favourite circuit.
Despite the charge to the title, with a 77 point lead over Ferrari's Fernando Alonso with five races remaining, Vettel and the British-based team have taken it one race at a time and stayed rigorously focused.
Horner recognised however that winning four successive crowns would be a massive achievement.
"To win it in the first place was special and to defend it was remarkable," he said. "And we never thought we could surpass the triple. If we get a fourth it would be something the whole team would be blown away by. It would be phenomenal."
Vettel has enjoyed the best car on the grid, making the most of acclaimed designer Adrian Newey's creations, but Horner said the youngster was also reaping the rewards for putting in the hard yards.
"He is only just 26 years of age but he works so hard at it," he said. "What you guys don't see is behind the scenes how much effort he puts into his preparation, his training.
"He is hugely self-critical. He is always looking at areas where he can improve, where he can be better.
"He'll look at this race and look at things that he could be stronger in. It is that inward-looking that he has that keeps propelling and driving him forward. I think he has got the balance about right."
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- Juan Manuel Fangio
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