Why is 4...b5 an Error in Ruy Lopez?

I guess I shouldn't weigh in on an old thread. But this sort of thing is something I've noticed in databases before, and I don't think people have pinpointed the real reason yet.

My hunch is that the real cause is simply that that the people who play 4 ... b5 are weaker players. They don't lose because they played 4 ... b5 (which is objectively fine). It's the revserse. They played 4 ... b5 because they don't quite understand the concepts and move orders of the Ruy, and because they don't quite understand the Ruy, they later went on to make other mistakes and lose on account of them.

People who know the Ruy well understand that ... b5 can be delayed and that delaying it gives you just an ounce more flexibility. Objectively, it there's no way for White to take advantave of any extra options, but strong players know better than to even allow those extra options. It's true that a few (there was a list of strong players who have done this) simply know all the options well enough that they can get away with this and may even do it to try to mess with people, but my guess is that's a minority of games in this line, and that in many cases, Black is simply a weaker player.

Occasionally I've noticed other database lines where this seems to be the issue. One side plays an unnecesarily imprecise move order that isn't objectively bad but scoresbadly because better players would have known not to allow their opponent extra options.