There's nothing wrong with an immediate b5, and there's only one move for you to reply with. Where's the problem?
The only way to avoid it is to stop playing the Ruy Lopez, and you can't capitalise because it isn't weak.
Normally b5 isn't normally immediate simply because white isn't really threatening to win a pawn until he supports his own rook.
I know I only have one response. It was just one of those things were you don't see 4. . b5 in any of the intros. to the Ruy that you see at my level (Idiot's guide to openings, etc.) Thought there might be something I was missing that better players use in response to make that an unpopular line at higher levels.
At your level, you could try meeting 4. ... b5 with:
So I've been playing the Ruy Lopez as white in a run of recent games and have been running into trouble with this line which frequently occurs at my level of play.
I know 3 . . a6 4. Ba4 is the Morphy Defense. I also know, I think, that black usually doesn't follow with b5 (could be wrong). Unfortunately, at my level, they do and it's been causing me problems.
Any suggestions on how to either avoid this situation or to capitalize as white in some way that I just don't see.
Note: this is not a current position in any of my live games, but, rather, something I encountered 4 or so times in the last month.
Thanks -
CMI