A 2x4 is 2x4
In the United States, softwoods like pine and redwood are sold by their "nominal" dimensions, while the actual dimensions for woods up to 7" is 1/2" smaller to account for planing and drying. So a pine board sold in the US as a 2x4 has a minimum size of 1 1/2" by 3 1/2".
This according to the woodworking FAQ at
ftp://ftp.cs.rochester.edu/pub/archives/rec.woodworking/woodwork-faq
. According to the FAQ, the corellation between the nominal and actual size of hardwoods sold in the US is even more complex. Weird.