Quadrennial can either mean lasting for four years, or recurring every four years. That's rather confusing, isn't it? Suppose I told you I had a cake eating binge on a quadrennial basis - would that mean I stuffed myself silly for forty-eight consecutive months, or that I lasted almost four years between blow-outs?
Now you're confused over the meaning of a word which you hadn't previously even knew existed. There's no need to thank me. I do this kind of thing out the goodness of my heart.