> The article mentions ‘ four years ago is unacceptable so it sounds like they want to retest new versions every four years, rather than every new version.

Your choice of quote makes it seem like you are misunderstanding or deliberately misrepresenting the article. In more context:

> "As we've said before, trials from four years ago conducted in people without natural immunity no longer suffice. A four-year-old trial is also not a blank check for new vaccines each year without clinical trial data, unlike the flu shot which has been tried and tested for more than 80 years," Nixon said in a statement he had earlier sent to The Washington Post. "The public deserves transparency and gold-standard science — especially with evolving products."

This states that a Covid vaccine passing the placebo-controlled study requirement 4 years ago will not suffice to accept updated versions of the same Covid vaccine -- not that vaccines and/or delivery mechanisms will only need to be tested every 4 years. More concisely: it's an upper bound, but not a lower bound.

Edit: Fixing up some grammar.

> This states that a Covid vaccine passing the placebo-controlled study requirement 4 years ago will not suffice to accept updated versions of the same Covid vaccine

Yes. That is what the comment you were replying to states.

> More concisely: it's an upper bound, but not a lower bound.

You can’t say it’s any bound at all. Maybe more than two milliseconds old (less than four years) is unacceptable, maybe 16 millennia (more than four years) is unacceptable. They’re just thinking about four years as being unacceptable. Which sounds reasonable.