  The 4MOST consortium is currently halfway through a Conceptual Design study
for ESO with the aim to develop a wide-field (>3 square degree, goal >5 square
degree), high-multiplex (>1500 fibres, goal 3000 fibres) spectroscopic survey
facility for an ESO 4m-class telescope (VISTA). 4MOST will run permanently on
the telescope to perform a 5 year public survey yielding more than 20 million
spectra at resolution R~5000 ({\lambda}=390-1000 nm) and more than 2 million
spectra at R~20,000 (395-456.5 nm & 587-673 nm). The 4MOST design is especially
intended to complement three key all-sky, space-based observatories of prime
European interest: Gaia, eROSITA and Euclid. Initial design and performance
estimates for the wide-field corrector concepts are presented. We consider two
fibre positioner concepts, a well-known Phi-Theta system and a new R-Theta
concept with a large patrol area. The spectrographs are fixed configuration
two-arm spectrographs, with dedicated spectrographs for the high- and
low-resolution. A full facility simulator is being developed to guide trade-off
decisions regarding the optimal field-of-view, number of fibres needed, and the
relative fraction of high-to-low resolution fibres. Mock catalogues with
template spectra from seven Design Reference Surveys are simulated to verify
the science requirements of 4MOST. The 4MOST consortium aims to deliver the
full 4MOST facility by the end of 2018 and start delivering high-level data
products for both consortium and ESO community targets a year later with yearly
increments.
