4140 is the brittle one. That's why it is prone to cracking. When you try to pull 4140 apart it cracks because of its inability to stretch or flex.This is the same problem you have with a stock crank.
Because of this,the list of applications for 4140,in the Earle M. Jorgensen Company's alloy reference book doesn't include crankshafts. Here is what they have to say about 4340.
4340
Nickel-Chromium-Molybdenum Steel
This is the “king” of the hardening grades of constructional alloy steels. Because
of richer alloy content, 4340 possesses much deeper harden ability than the 4100
series. This advantage is realized principally where high strength is required in
heavy sections. In addition, unusually high harden ability insures maximum toughness
and ductility at the desired strength level. The fatigue-tensile ratio makes this grade
ideal for highly stressed parts. It maintains its strength, ductility, and toughness
at relatively high temperatures. It has remarkable non-distorting properties for an
alloy steel. Thus, for high strength in heavy sections, or for highly stressed parts
operating under the most severe conditions, or where the greatest margin of safety
is desired, this is the steel to use.
Here is there list of applications.
APPLICATIONS – Automotive and marine crank shafts, diesel engine crank shafts
and other heavy-duty shafting, axle shafts, dies, subs, gear shafts, gears,
mine-drilling parts, oil-well fishing tools, perforating gun bodies, master hobs, etc.