Neutron diffraction experiments have been carried out to characterize the magnetic structures and order parameters in an intermediate phase of NdB4 showing the successive phase transitions at T-0 = 17.2 K, T-N1 = 7.0 K, and T-N2 = 4.8 K. We have revealed the antiferromagnetic ordering with the propagation vectors q(0) = (0, 0, 0), q(0) and q(s1) = (delta, delta, 0.4) (delta similar to 0.14), and q(0) and q(s2) = (0.2, 0,0.4) in phase II (T-N1 < T < T-0), phase III (T-N2 < T < T-N1), and phase IV (T < T-N2), respectively. The observed patterns in phase II are successfully explained by postulating a coplanar structure with static magnetic moments in the tetragonal ab-plane. We have found that the magnetic structure in phase II can be uniquely determined to be a linear combination of antiferromagnetic "all-in/all-out"-type (Gamma(4)) and "vortex"-type (Gamma(2)) structures, consisting of a Gamma(4) main component (77%) with a small amplitude of Gamma(2) (23%). We propose that the quadrupolar interaction holds the key to stabilizing the noncollinear magnetic structure and quadrupolar order. Here, the frustration in the Shastry-Sutherland lattice would play an essential role in suppressing the dominance of the magnetic interaction.