We makes an explicit description of compact 4-dimensional nilmanifolds as principal torus bundles and show that they are sysmplectic. We discuss some consequences of this and give in particular a Seibebrg-Witten-invariant proof of a Grovmov-Lawson theorem that if a compact 4-dimensional nilmanifold admits a metric of zero scalar curvature, then it is diffeomorphic to 4-tours, $T^4$.