On April 4, 1949, Canada became a founding member of the North Atlantic Treaty Organization (NATO) by signing the North Atlantic Treaty. This military and political alliance was established to safeguard the freedom and security of its member countries through collective defense. ​Wikipedia+2Government of Canada+2The Canada Guide+2

Additionally, on April 4, 1942, during World War II, a Royal Canadian Air Force plane spotted the Japanese fleet en route to Ceylon (now Sri Lanka). The timely warning allowed for the successful defense of the island, an event Winston Churchill referred to as “the most dangerous moment of the war.” ​Veterans Affairs Canada

Furthermore, on April 4, 1986, the town of Port Simpson in British Columbia was officially renamed Lax Kw’alaams, reflecting its Indigenous heritage. ​Wikipedia

These events highlight significant moments in Canada’s history that occurred on April 4.