Under the same quenching and tempering process, 4340 can achieve a higher ultimate hardness, meaning 4340 steel is harder than 4140 steel.
However, in practical applications, material selection is not simply a matter of “which is harder,” but rather should consider:
While meeting the necessary hardness/strength requirements, does the material also need higher toughness to withstand impacts? Does it need better fatigue performance to ensure lifespan? Furthermore, practical constraints such as cost, processing difficulty, and cross-sectional dimensions must also be considered.