All three are considered chro-moly. All three require full heat treating to obtain the desired properties of high hardness and yield strength. By full heat treating I mean heating, quenching, and tempering, also called `quench and temper`. These three vary in their response to heat treatment. 
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4340 is one of the great materials of all time, and is used for the best available connecting rods and other highly stressed parts. It would be perfect for bolt on spindles, but the other grades are good too.  4340 has .4 % carbon and a lot of alloying elements, and it is `highly` hardenable. It will reach full hardness with a relatively gentle quenching process such as hot oil or salt bath, so it reaches max strength with less quench distortion. 
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4130 has .3% carbon, and fewer alloying elements, so is easier to weld and machine but has `low hardenability`. As a result it requires a more brutal water quench to reach full strength. If you are not going to heat treat, you might as well use the 4130 for the ease of machining. 4130 was developed in the 1930`s for gas welding of aircraft structures, and is intended to develop decent strength from normalizing. Normalizing is heating to full hardening temp and then air cooling rather than quenching. So 4130 develops good strength without the cracking and distortion that can result from water quenching. Besides it had to be developed for normalizing, as there are few if any quenching tanks big enough for an aircraft fusilage.
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4140 is also highly hardenable, but probably the most difficult of the three to machine. 
