I've long wanted to write a forum post about this kind of attacks and how to meet them. Unfortunately, I'm overloaded with real life work, so I never got to it.
Tiger recommends 4...a6 in any case and 5...Nf6 against h4. In fast games, White often then just pushes the h-pawn, sacs the rook for the knight and tries to attack the uncastled king, but if we're talking about serious chess, things are different. ...b5 and ...Nd7 are played almost invariantly, but the reaction towards a somewhat later h4 depends on other factors. ...h5 and ...h6 are both ok. I would usually prefer ...h6 because if White continues with g4, it's possible to lock the ks pawns.
Basically, this setup with its subvariations is covered in TMT chapter 3. I'll try to compile a condensed summary of the mainlines as soon as I find some time.
and we discuss the early opening stage often very thoroughly.
Recently I've been facing similar set ups where white tries to attack via Bh6 castling long and pushing the h pawn etc. What's the safest method of defending against such attacks