IPA: /kwɔːtəˈsɛntənəri/
KK: /kwɔːtərˈsɛntənɛri/
A celebration or event marking the 400th anniversary of something.
The city held a grand festival for its quatercentenary, celebrating 400 years of history.
Quatercentenary → It is formed from "quater" (from Latin "quater", meaning four times) and "centenary" (from Latin "centenarius", meaning a hundred). The word refers to a celebration or event occurring every four hundred years.
Think of 'four times' ('quater') and 'a hundred' ('centenary') to remember that a quatercentenary is a celebration that happens every four hundred years.