We report the generation of continuous-wave (cw) tunable mid-infrared (mid-IR) radiation across 4608-4694 nm with multi-tens of mW level output power using difference-frequency-generation (DFG) in the new nonlinear material, orientation-patterned gallium phosphide (OP-GaP). A 40-mm-long OP-GaP (Λ=85.1 μm) is used for DFG, pumped with a cw Tm-fiber laser and a home-built cw Yb-fiber-pumped MgO:PPLN-based optical parametric oscillator (OPO). By varying MgO:PPLN crystal temperature and simultaneously adjusting the phase-matching temperature in OP-GaP, with an input pump power of 34 W at 2010 nm and OPO idler power of >2.9 W across 3505-3557 nm, we achieved DFG tuning across 4608-4694 nm, providing >30 mW of output power across 96% of the full tuning range with 43 mW at 4608 nm. The output at 4608 nm exhibits high beam quality, with a passive power stability of 2.5% rms over 1.5 mins. The performance of the system at high pump powers, temperature acceptance bandwidth, DFG spectral characteristics, and spatial quality has been investigated.