Article Four states: “The parties shall consult each other if, in the opinion of either party, there is a threat to the integrity of the territory, to the political independence or to the security of either party.” This does not have to result in concrete reactions.
According to NATO, the article has been invoked seven times since the alliance was founded in 1949. This was last the case on February 24, 2022. Bulgaria, Estonia, Latvia, Lithuania, Poland, Romania, Slovakia and the Czech Republic requested this after the Russian invasion of Ukraine.
Article Five provides that the Allies regard an armed attack against one or more of them as an attack against all of them. You agree to provide assistance. Specifically, it states that it is about the measures they consider necessary to restore and maintain the security of the North Atlantic area – including armed force.
Article Five has only been activated once in the face of NATO, after the attacks of September 11, 2001. This led to Germany and other NATO countries joining the war against the Taliban and the terrorist organization Al Qaeda in Afghanistan . However, the wording of the article does not indicate that NATO countries have to send their own troops to provide support, for example. It only obliges you to take immediate measures that the respective NATO state deems necessary.