GM Eugene Perelshteyn comes up with a new weapon in our beloved Grand Prix Attack 5.a4! Eugene explains the power of this waiting vs the most common reply 5…e6. The key idea is 6.e5! and Black is almost busted!
Against 1. e4 c5 2.Nc3 e6 3.f4 d5 4. Nf3 Nf6, I think 5.Bb5+ Bd7 6.Bxd7+ Nxd7 7.d3 should be recommended, when after 7.dxe4 8.Nxe4 transposes. 5.e5 Nfd7 6.g3!? is also interesting.
Alex Latham says
Against 1. e4 c5 2.Nc3 e6 3.f4 d5 4. Nf3 Nf6, I think 5.Bb5+ Bd7 6.Bxd7+ Nxd7 7.d3 should be recommended, when after 7.dxe4 8.Nxe4 transposes. 5.e5 Nfd7 6.g3!? is also interesting.