I don't remember which of the chess books defined it this way, sorry, but I have seen a backward pawn defined as a pawn where the pawns on either side of it are ahead of it or missing. Although if both are missing it is an isolated pawn not a backward pawn.
Using this definition at the start of the game no pawns are backward. In the example none of the pawns are backward since they all have a neighbor pawn on one side or the other that is not ahead of them. Backward pawns are weak points precisely for the reasons mentioned above, requiring another piece to defend them.
In the example if instead of 4.d4 white played 4.Bxc6+ and back responded by playing 4. .. bxc6 then the pawn on c7 would become a backward pawn, and a doubled pawn since there are two pawns on the same file.
In the example as played if black responds 4. .. g6 then he would create 2 backward pawns on f7 and h7. Either of them could stop being backward by advancing though.