Current 4340 Steel Hex Bar prices by dimension, with $/lb for cross-size comparison.
About 4340|When
4140 isn't strong enough. The addition of 1.65-2.00% nickel on top of the chromium and molybdenum gives 4340 significantly better toughness, impact resistance, and hardenability, especially in thick cross-sections. This is aircraft landing gear steel, heavy-duty crankshaft steel, drill collar steel. At the same hardness as 4140, the two have similar tensile strength, but 4340 retains far more toughness and fracture resistance. The nickel makes it through-harden more uniformly in large sections where 4140 would go soft in the core. Harder to machine than 4140 (about 55-60% machinability vs 65% for 4140). Costs roughly double. Use 4340 when you need the toughness or the section size demands it. Otherwise 4140 does the job for less money. Compare to 4140 (cheaper, easier to machine, adequate for smaller sections) and
4330V (vacuum-melt aerospace variant).