4MOST, the 4m Multi Object Spectroscopic Telescope, is an upcoming optical, fibre-fed, MOS facility for the VISTA telescope at ESO's Paranal Observatory in Chile. Its main science drivers are in the fields of galactic archeology, highenergy physics, galaxy evolution and cosmology. The preliminary design of 4MOST features 2436 fibres split into lowresolution (1624 fibres, 370-950 nm, R > 4000) and high-resolution spectrographs (812 fibres, three arms, ~44-69 nm coverage each, R >18000) with a fibre positioner and covering an hexagonal field of view of ~4.1 deg2. The 4MOST consortium consists of several institutes in Europe and Australia under leadership of the Leibniz-Institut fur Astrophysik, Potsdam (AIP). 4MOST is currently in its Preliminary Design Phase with an expected start of science operations in 2021. Two third of fibres go to two Low Resolution Spectrographs with three channels per spectrograph. Each low resolution spectrograph is composed of 812 scientific and 10 calibration fibres using 85μm core fibres at f/3, a 200mm beam for an off-axis collimator associated to its Schmidt corrector, 3 arms with f/1.73 cameras and standard 6k x 6k 15μm pixel detectors. CRAL has the responsibility of the Low Resolution Spectrographs. In this paper, the optical design and performances of 4MOST Low Resolution Spectrograph designed for 4MOST PDR in June, 2016 will be presented. Special emphasis will be put on the Low Resolution Spectrograph system budget and performance analysis.
