4140 vs 4340 3

There is a huge difference between these two alloys chemically, namely the nickel content: 4140 nominally has no Ni and 4340 contains between 1.65 and 2.00 % Ni. This contributes to substantial differences in hardenability and fracture toughness, and also has a positive effect on distortion during quenching and corrosion resistance. As a surrogate for K1c values, I'll use Izod impact energy as a comparison of the relative toughness of these two alloys:

4140 / 4340
tensile strength in MPa impact energy in Joules
~1965 (tempered @205C) 15 / 20
~1720 (tempered @315C) 9 / 14
~1020 (temp. @595/650) 93 / 100

This data is for room temperature. 4340 will be MUCH tougher at lower temperatures. 2% Ni shifts the ductile to brittle transition temperature appreciably-- for low carbon steels it would be a delta of ~ 100 degrees F, for medium carbon steels it is less.

Having said all of this, these alloys can be used for many of the same applications. If the application does not require the higher level of fracture toughness (especially at low temperatures) that 4340 offers, then 4140 can be used as a direct replacement, with the caveat that distortion during heat treatment will be higher and thicker sections will not quench to the same hardness. I would caution you against ever saying that an alloy is suitable or unsuitable for a generic appplication without fully understanding the operating environment (forces, temperature, corrosion, etc.). You may want to provide some additional details about the types of components you manufacture, their operating environments, etc. so that we can better answer your question.