The main line move order of the g4 move.
Which is known as the Shirov Gambit.
I don't know what the best black response is after 5.g4.
However, the moves 5...h6 and 5...g6 both have been played and seem to be doing fairly ok for black in stats compared to the move 5...Nxg4 which seems to be in whites favor.
The only real reason to play the Pirc Move order would be to try and trick white.
Black uses the Pirc Move order to trick white into playing the Philidor Defence/Hanham Variation while avoiding the 4.Bc4 lines.
If I use the stand move order of the Philidor you will see what I mean.
In the below Diagram.
When Black plays the Pirc Move order it tricks white.
White gets tricked because white believes black is playing the Pirc.
In which case white will respond how they normally respond against the Pirc.
Which black is playing the Pirc but they will tranpose out of it.
As you can see and read from the above diagram.
After black plays 3...e5
White is faced with a problem of what to do.
White has often gone ahead and played.
4.Nf3 which I will show you in the below diagram.
You might ask the question's:
Why does black try to trick white?
To avoid the 4.Bc4 lines
What is so bad about the 4.Bc4 line which is causing black to do a move order trick?
I am not Philidor Defence expert, but I believe the issue is white can try and do some very annoying forcing lines.
Which in some cases result in the loss of a pawn.
In other cases it results in the lost of the bishop pair.
I will not explained all the above moves.
However, you can see how they are annoying and end up like the text I said in red.
If black doesn't play 4...c6
He can end up in some matting patterns similar to the ones Skotheim showed in his one diagram.