4GLS is a superconducting energy recovery linac (ERL) with output energy around 600 MeV, delivering CW beam currents up to 100 mA or alternatively high charge bunches for FEL applications. Production of short (fs) bunches is a vital part of the specification and will require a flexible beam transport system incorporating various compression stages. In addition to the undulator beam lines with sources in the ERL recirculation path there will be three FELs; two will be oscillator types in the infared and VUV respectively, and the third will be a high gain system for XUV output. The talk outlines the project, its current status and the R&D challenges that it poses.


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