The goal of this survey is to study the formation and evolution of the Milky Way halo to deduce its assembly history and the 3D distribution of mass in the Milky Way. The combination of multi-band photometry, Gaia proper motion and parallax data, and radial velocities and the metallicity and elemental abundances obtained from low-resolution spectra of halo giants with 4MOST, will yield an unprecedented characterisation of the Milky Way halo and its interface with the thick disc. The survey will produce a volume- and magnitude- limited complete sample of giant stars in the halo. It will cover at least 10 000 square degrees of high Galactic latitude, and measure line-of-sight velocities with a precision of 1-2 km s^-1 as well as metallicities to within 0.2 dex.