4GLS is a suite of accelerator-based light sources planned to provide state-of-the-art radiation in the THz-soft X-ray regime. Superconducting energy recovery linac (ERL) technology will be utilised in combination with free electron lasers (IR to XUV), undulators and bending magnets. The undulators will generate spontaneous high flux, high brightness synchrotron radiation (SR), of variable polarisation from 3 - 800 eV, optimised in the lower harmonics up to about 200 eV. Viable radiation at energies up to several keV may be provided from multipole wiggler magnet radiation. The ERL technology of 4GLS will allow shorter bunches and higher peak photon fluxes than is possible from storage ring sources. VUV and XUV FELs will be used to generate fs-regime pulses that are broadly tuneable and more than a million times more intense than the equivalent SR. 4GLS will probe ultra-fast dynamics in a wide range of fields. Pump-probe experiments will allow the study of chemical reactions and short-lived intermediates on the timescale of bond formation, even for very dilute species. Circularly polarised light sources in a variety of frequency regimes will be used to manipulate and monitor carrier charge and spin transport in device structures. The high intensity of the FEL radiation will allow high resolution in imaging and the opportunity to probe nonlinear regimes. The lower intensity, high repetition rate ERL SR provides ideal sources for ultra-high energy resolution spectroscopy, especially in the solid state. Around £22 M funding has been obtained for the first stages of the project (construction of a prototype source, science demonstrations and design study work, currently nearing completion at STFC Daresbury Laboratory). It is anticipated that the full 4GLS facility will be available to users in 2014.

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