You should investigate the Gurgenidze system as well, its a common way for Black players to avoid the Austrian and is not trivial to play:

1.e4 g6 2.d4 Bg7 3.Nc3 c6 4.f4 d5 etc.

Playing the Pirc/Modern sometimes with Black myself I second Stigma´s observation.

As a sidenote if you rely on Khalifman´s book only you should be aware that his recommendation against 1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.f4 Bg7 5.Nf3 c5 is considered fairly comfortable for Black lately.

I've been using Khalifman's openings according to anand series for my white repertoire.  Sometimes what I do is I take a week and try to go over one defense for black.  However I ran into a problem with his Volume 4 which encompasses the modern and pirc, ...there are too many possible choices.  The book is 402 pages long. 

  What I've done so far is, after a tournament game where the modern/pirc is employed, I look up the line played in Khalifman's book.  It felt like a waste of time to try to go through the whole book.

Anyhow, my question is this...What are the most common/popular  modern/pirc lines played against the austrian attack setup(pawns on d4/e4/and f4)? 

Looking at the databases to answer this question for me hasn't helped because of the transpositional possibilities.