On a system as critical as the brakes, I highly doubt they yolo just guessed and said, eh, just make it 4 years, that sounds good. I find it far more believable they did extensive testing of the brake system, which they build, and thus know just how much water is and isn't getting into the system, and what a good fluid replacement interval is.
Or maybe everyone that works there is an incompetent monkey and they never test anything, ever. It's probably that, definitely. That's also why we see so many stories in the news about 5 year old Tesla's brakes failing. Given that we all know how long full self driving has been promised, I'm sure we'll get those news stories any day now.
