It is a bit premature. It is not losing or anything but white's bishop is better on b3 than on a4. b5 is not a developing move. The point of the move is to break the pin if white doesn't go for exchange variation, but there is no rush to do so. After d4 by white black needs to be careful. Look at this for instance:
Of course, it will not always happen like this. Black is fine, he can play like that.
Ruy Lopez -- Why is 4...b5 not more popular?
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Apr 1, 2021
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In trying to understand the Ruy Lopez more, and looking through the opening database, I'm curious about what's "wrong" with after 4.Ba4: 4....b5. The analyzer has it just about at the same level as other common Black's 4th moves, but when you look at the Opening Database, it's see's very little play.
I see that far an away the most common response to 4.Ba4 is Nf6, developing the knight. Wouldn't it make sense to play your b-Pawn and give chase to the Bishop THEN develop your knight?
What am I not seeing?
Thanks!