Deep K-band imaging of the most luminous z≃ 4 quasars currently offers the earliest possible view of the mass-dominant stellar populations of the host galaxies which house the first supermassive black holes in the Universe. This is because, until the advent of the James Webb Space Telescope, it is not possible to obtain the necessary deep, sub-arcsec resolution imaging at rest-frame wavelengths λrest > 4000 Å at any higher redshift. We here present and analyse the deepest, high-quality KS-band images ever obtained of luminous quasars at z≃ 4, in an attempt to determine the basic properties of their host galaxies less than 1 Gyr after the first recorded appearance of black holes with Mbh > 109 M⊙. To maximize the robustness of our results, we have carefully selected two Sloan Digital Sky Survey quasars at z≃ 4. With absolute magnitudes Mi < −28, these quasars are representative of the most luminous quasars known at this epoch, but they also, crucially, lie within 40 arcsec of comparably bright foreground stars (required for accurate point spread function definition), and have redshifts which ensure line-free KS-band imaging. The data were obtained in excellent seeing conditions (<0.4 arcsec) at the European Southern Observatory on the Very Large Telescope with integration times of ≃5.5 h per source. Via carefully controlled separation of host galaxy and nuclear light, we estimate the luminosities and stellar masses of the host galaxies, and set constraints on their half-light radii. The apparent KS-band magnitudes of the quasar host galaxies are consistent with those of luminous radio galaxies at comparable redshifts, suggesting that these quasar hosts are also among the most massive galaxies in existence at this epoch. However, the quasar hosts are a factor ∼5 smaller (〈r1/2〉= 1.8 kpc) than the host galaxies of luminous low-redshift quasars. We estimate the stellar masses of the z≃ 4 host galaxies to lie in the range 2–10 × 1011 M⊙, and use the C iv emission line in the Sloan optical spectra to estimate the masses of their central supermassive black holes. The results imply a black hole-to-host-galaxy mass ratio Mbh:Mgal≃ 0.01–0.05. This is an order of magnitude higher than typically seen in the low-redshift Universe, and is consistent with existing evidence for a systematic growth in this mass ratio with increasing redshift [i.e. Mbh:Mgal∝ (1 +z)1.4−2.0], at least for objects selected as powerful active galactic nuclei.