You're assuming that this one is a three-year La Niña. You'll know whether this is a 3- or ≥4-year La Niña in about 18 months. The warmest seven years since records began have all been since 2015, so I wouldn't bet against a La Niña record now either.
That would be a a very big surprise. But I could say "don't assume that today is the last day of this extremely hot weather, it might go on until tomorrow or even longer" and that would be much less remarkable. You don't know the end of an ongoing event until it's past.
It is how probabilities work. A plain example: The odds of flipping ten heads in a row are 1/1024, but if you've already flipped nine heads, the odds of flipping ten heads are 1/2.
It's not the odds of the fourth flip, it's the difference between four flips and a fourth flip.
Look upstream, you'll find someone (you perhaps? I don't care who) who doesn't realise whether the current La Niña situation is equivalent to a fourth-flip or a four-flips situation. A common problem in statistics, in my experience: People sort of understand two statistical statements, but then apply the wrong one to the real-world situation they're in.
