The 4MOST consortium aims to provide the ESO community with a fibre-fed spectroscopic survey facility on a 4m-class telescope with a large enough field-of-view (FoV) to survey a large fraction of the southern sky in a few years, a multiplex and spectral resolution high enough to detect chemical and kinematic substructure in the stellar halo, bulge and thin and thick discs of the Milky Way, and enough wavelength coverage (> 1.5 octave) to secure velocities of extra-galactic objects over a large range in redshift. 4MOST will run permanently on the selected telescope to perform a 5 year public survey yielding more than 7 million (goal > 25 million) spectra at resolution R ∼ 5000 and more than 1 million spectra at R 20, 000. Such an exceptional facility enables many science goals, but our design is especially intended to complement three key all-sky, space-based observatories of prime European interest: Gaia, eROSITA and Euclid.