  Rare-earth-doped fiber lasers are promising contenders in the development of
spectroscopy, free-space communications and countermeasure applications in the
3 - 5 {\mu}m spectral region. However, given the limited transparency of the
commonly used fluorozirconate glass fiber, these systems have only achieved
wavelength coverage up to 3.8 {\mu}m, hence fueling the development of more
suitable fiber glass compositions. To this extent, we propose in this Letter a
novel heavily holmium-doped fluoroindate fiber, providing extended transparency
up to 5 {\mu}m, to demonstrate the longest wavelength room temperature fiber
laser at 3.92 {\mu}m. Achieving $\approx$ 200 mW of output power when cladding
pumped by a commercial 888 nm laser diode, this demonstration paves the way for
powerful mid-infrared fiber lasers emitting at and beyond 4 {\mu}m.
