The Four Pawns Attack is a variation of the King's Indian Defence and begins with the moves:1.d4 Nf6
White creates a large pawn-center and prepares a future breakthrough with e5. After pieces are developed Black tries to weaken the white pawn formation by pawn advances e7-e5, c7-c5 or b7-b5.
Modern line of play is: 5.f4 O-O 6.Nf3 Na6!? followed by a pawn sacrifice with 7...e5 which will create favorable tactical possibilities for Black. (see diagramm below)
The main line is:
5.f4 O-O 6.Nf3 c5 7.d5 Black can now sacrifice a pawn by 7...b5 in exchange for active counterplay (see diagramm below)
or can play the usual 7...e6 followed by 8...exd 9.cxd exchangeing pawns to open the e-file.
After that there is the 9...b5 Variation (see diagramm below).
the 9...Re8 Variation (see diagramm below).
and the 9...Bg4 Variation (see diagramm below).
If you have White you should know all those variations especially the 6...Na6 Variation which is very tricky. You can replay those variations below. They are stored in the same order as shown above.
I believe that Black should not play the 9...b5 Variation and avoid the 9...Re8-Variation as White scores very well here. I have played those variations myself many times as White and I think that White got higher winning chances here. This assumption is confirmed by my database as well.
If you have Black then I recommend the 9...Bg4 Variation. In my opinion White can achieve nothing here as the knight on f3 will be exchanged and this stops White to make the e5 break. If Black plays the c-pawn to c4 and plays his knight to the outpost c5 then White will have a hard time not to lose.
Replay the variations below and form your opinion. First there are two testgames which are completely different. After that the order is game number: 3 > 6...Na6 games (modern line), 66 > 7...b5 games, 113 > 9...b5 games (after exchanging pawns), 131 > 9...Re8 games and 166 > 9...Bg4 games.