For developing an ideal efficient and low-threshold medium for fiber lasers operated at 4 μm wavebands, 0.05 wt.% to 0.5 wt.% Dy3+ ions doped Ga0.8As39.2S60 chalcogenide glasses were investigated and a 4.2 μm fiber laser was theoretically studied based on the rate and propagation equations. It was shown that the Ga0.8As39.2S60 glass shows a desirable large Dy3+ ion solubility, which has been increased by an order of magnitude compared to As2S3 glass. Dy-rich nanocrystallines were found when the Dy3+ ions' concentration is more than 0.3wt.% (i.e. 3000 ppmw), and the concentration quenching was found based on spectral analysis; however, no rapid decreases in lifetimes were observed. 0.3wt.% Dy3+ doped Ga0.8As39.2S60 glass possesses relatively large laser quality factor σemi × τmea = 1.70 × 10-23 cm2·s and excellent thermal stability (ΔT = 182°C), which can be successfully drawn into fibers indicating good potential for mid-infrared fiber laser. © 2018 Optical Society of America.