"I think it would be better, I think it would liven things up...It would be refreshing," the 74-year-old Scot told reporters at a lunch to preview this month's Motorsport Hall of Fame event in London.
"Four times is a bit greedy," he added, jokingly.
Vettel, 26, became the youngest quadruple champion last season and won the final nine races of the year. Nobody has beaten him since July.
Red Bull have also won the last four constructors' titles and will start the new season in March as favourites again.
Fellow-German Michael Schumacher, who retired in 2012 at the age of 43 and is currently in a coma after a skiing accident in France last month, won five titles in a row with Ferrari between 2000 and 2004.
"Nobody is up there forever," said Stewart. "The world moves on and I'm sure that will be the case. I don't know if it will be next season.
"Red Bull are pretty impressive and they've now got more money coming in from sponsors than they have from Red Bull, because they are getting TV."
This season sees a new V6 turbocharged engine replacing the old V8s, with fuel economy and driving style much more significant factors, and reliability will be a bigger concern.
Red Bull boss Christian Horner warned earlier on Wednesday that the opening race in Australia could see a significant number of retirements, perhaps as much as half the field, instead of most cars finishing.
It should also be harder for Vettel to run away with the championship following a controversial decision to award double points at the final race in Abu Dhabi in a bid to keep the title open for longer.
Stewart expects last year's runner-up and past double champion Fernando Alonso to get the better of team mate Kimi Raikkonen, the 2007 world champion, at Ferrari this season when they are paired for the first time.
Lewis Hamilton, the 2008 champion with McLaren and a winner for Mercedes last year, is another favourite even if questions had still to be answered about his consistency over the course of a year.
"I think Lewis is the fastest driver in the business," said Stewart. "Just sheer speed. When he puts a quick one in, he's the master at it."
The Scot said he was sure McLaren's Jenson Button, the 2009 champion, would not let grief at the death last weekend of his father John - a constant presence at races and a hugely popular paddock figure - affect his racing.
"A racing driver is a peculiar animal," he said.
"Look at all the death that we had and yet for whatever reason we still raced...I think racing drivers probably are tougher than most people.
"John lived the fullest of lives, and he had a heart attack and he was 70 years of age. And there wasn't much life that he'd missed."
(Editing by Ed Osmond)
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