As Hoacin gives in his answer, after 1.e4 d6 2.d4 Nf6 3.Nc3 Nbd7, both 4.f4 and 4.g4 are strong moves. They are not only aggressive, but also objectively speaking one of the best options in the position.
Another way to spice up the game is the Shirov Gambit: 4.Nf3 e5 5.g4. If I'm not mistaken, he was the very first one to play this remarkable pawn sacrifice: against GM Azmaiparashvili in 2003. Later, GM Shirov used it again, winning some nice miniatures against IM Klinova and GM Cyborowski.
According to the Game Database of ChessTempo, 5.g4 is the second most popular move, after 5.Bc4.
There is analysis material on the gambit available, for example an article written by GM Gormally.
One of the advantages of the Shirov Gambit is that you can play it against two popular move orders that lead to the Philidor: 3....Nbd7, 4....e5 and 3....e5, 4....Nbd7.
1.e4 d6 2.d4 Nf6 3.Nc3 Nbd7 (3...e5 4.Nf3 Nbd7) 4.Nf3 (4.f4)(4.g4) e5 5.g4 (5.Bc4) Nxg4 6.Rg1 Ngf6 7.Bc4 h6 8.Be3