A novel mid-infrared fiber laser is reported based on an anti-resonant hollow-core fiber filled with carbon dioxide gas. A tunable 2-μm semiconductor laser amplified using a thulium-doped fiber is used to pump a low-loss anti-resonant hollow-core fiber with a length of 5 m, which is filled with carbon dioxide gas at low pressure. Particle beam inversion is responsible for obtaining the single-pass fiber laser output at 4.3 μm, which is the longest wavelength that has yet been reported for continuous wave fiber lasers at normal temperature, except for supercontinuum lasers. At a pressure of 500 Pa, the maximum laser output power of the R(30) absorption line is 82 mW and the slope efficiency is approximately 6.8% (relatively coupled pump power entering the hollow-core fiber), whereas the maximum laser output power of the R(28) absorption line is 63 mW and the slope efficiency is approximately 5%. This study provides a potentially effective pathway for obtaining compact and efficient 4-μm fiber gas lasers.