For the first time, to the best of our knowledge, the use of Nd3+ codoping for enhancing the ∼4.0 μm emission from the Ho3+:5I5 → 5I6 transition was investigated in a Ho3+, Nd3+-codoped Gd0.1Y0.9AlO3 [(Y,Gd)AlO3] crystal [Ho/Nd:(Y,Gd)AlO3]. In this study, the ∼4.0 μm emission characteristics and energy transfer were investigated in detail, and it was found that the codoped Nd3+ ions in the Ho/Nd:(Y,Gd)AlO3 crystal significantly enhanced the Ho3+:∼4.0 μm emission, depopulated the lower laser level of Ho3+:5I6, and had little effect on the higher laser level of Ho3+:5I5. It was also found that the energy transfer efficiency from Nd3+:4F3/2 to Ho3+:5I5 was as high as 43.0%, indicating that Nd3+ ions can be used as an effective sensitizer for Ho3+ ions and that Ho/Nd:(Y,Gd)AlO3 crystal has the potential to be pumped by a commercialized InGaAs laser diode (LD). These results suggest that Ho/Nd:(Y,Gd)AlO3 crystals are likely to become attractive hosts for developing solid-state lasers at around 4.0 μm under a conventional 808 nm LD pump.