The LS4 survey builds on the rich history of wide-field observations conducted by the La Silla Schmidt telescope. LS4 is upgrading the Quest camera to fully fill the Schmidt focal plane with 32 LBNL deep-depletion well CCDs to provide a total field of view of ~20 sq. deg. The LBNL CCDs are especially sensitive to red-optical and near-infrared photons, and LS4 will utilise a single “fixed” filter in which half the focal plane observes in the i band, one quarter observes in the g band, and one quarter observes in the z band.

The wide field of view and red-sensitive detectors will allow LS4 to monitor the night sky searching for stellar explosions, stellar variables, and anything else that changes brightness on timescales of minutes to years.

Scientific efforts within LS4 are organised via five science working groups:

• Multi-messenger Astronomy: LS4 will have a unique impact on multi-messenger astronomy (MMA) with its combination of wide-field, southern location and red sensitivity. Considering searches for gravitational wave and neutrino counterparts, LS4’s wide area will play a key role in mapping out the variable sky, and thus reject background variability for any searches for counterparts in southern MMA events. In addition, LS4’s wide area will be key in scanning poorly localised MMA events in regions with low galactic extinction, targeting fields of interest twice each night in g+i or i+z, allowing for >5000 sq. deg. to be scanned each night.
• Cosmology with Supernovae: The LS4 cosmology program seeks to use Type Ia supernovae and gravitationally-lensed supernovae to make advances in the measurement of the dark energy equation of state, tests of General Relativity through the growth rate of structure, and the Hubble constant. We advocate an LS4 survey in the ~20,000 sq. deg. of sky having low dust extinction between -80o < Dec < +20o. Interleaving the LS4 filters will allow for a 2 day cadence in i band and a 4 day cadence in g and z. This allows about half of the survey area to be monitored, with the window advancing eastward during the year. An unusually strong effort will be made to calibrate the supernova survey to a precision similar to that of the Roman Space Mission.
• Massive Black Holes: By surveying thousands of square degrees down to a depth of ~21 mag in g-band, LS4 is expected each year to discover around 20 tidal disruption events (TDEs) of stars by supermassive black holes. Surveying in multiple filters (g+i and i+z) will enable the selection of good candidates to follow up in real time with dedicated spectroscopic facilities in the Southern hemisphere. As well as providing an excellent data set for understanding TDE physics and enabling multi-wavelength follow-up, LS4 will also establish a baseline used to rule out prior AGN variability for some of the brighter TDEs that will be detected in future Rubin observations.
• Galactic Variables and Transients: LS4 will survey the Galactic bulge and plane to study the Milky Way’s population of stars, gas and dust, compact objects, and planets and probe Galactic structure in the inner Milky Way. This Galactic survey provides a multi-filter, time-domain view of the Milky Way that will enable us to find gravitational microlensing events, map the variable star population, search for rare binary systems that contain black holes, white dwarfs, and neutron stars, and monitor for Galactic novae, supernovae and other rare transients. With LS4’s wide field of view, we will observe the >1000 sq. deg. in the Galactic plane with nightly observations.
• Physics of Supernovae and Extragalactic Transients: Astrophysical transients are discovered by optical time-domain surveys in unprecedented numbers, with >20,000 new events in 2023 alone. This rate will grow exponentially as the Vera C. Rubin Observatory begins its 10 year Legacy Survey of Space and Time. LS4 will enhance this science by conducting an extragalactic transient survey using a unique g+i+z filter pattern capable of observing ∼9000 sq. deg. per night to a depth of ∼21 mag.  From these data, LS4 will provide unique constraints on young extragalactic transients discovered within days of explosion, populations of rare transients in the local Universe such as extreme supernovae, black hole-driven and more exotic transients, and probe the unique population of redder optical transients with LS4’s filter pattern and red sensitive CCDs.