Fourth-generation light sources, which would involve potentially higher brilliance, a higher degree of coherence, and ultrashort pulses, possibly at the hard X-ray region, be based on the self-amplified spontaneous emission principle of free electron lasers, would offer a further major advance in scientific capabilities. In this paper, the development history, the relationship with SASE FEL, the unique properties and the future applications of such sources are summarized and reviewed briefly.
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