Motivated by recent experimental results for GdRu2Si2 [Khanh, Nat. Nanotechnol. 15, 444 (2020)10.1038/s41565-020-0684-7], in which a nanometric square skyrmion lattice was observed, we propose a simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. Dipolar forces provide momentum-dependent biaxial anisotropy in reciprocal space. It is shown that in a tetragonal lattice, in the large part of the Brillouin zone, for mutually perpendicular modulation vectors in the ab plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, which leads to double-Q modulated spin structure stabilization. In the large part of its stability region, the latter turns out to be a square skyrmion lattice with a topological charge of ±1 per magnetic unit cell, which is determined by the frustrated exchange coupling and thus nanometer sized. Easy and middle axes can be swapped in the presence of additional single-ion easy-axis anisotropy. This results in the different phase diagram. It is argued that the latter case is relevant to GdRu2Si2.