Not true neccesarily. 4340 can be very very soft, in fact I took rockwell
B measurements today of 70/72 on 4340. It is extremely hardenable though, which typically allows for 4340 to acheive the martensitic reaction, thus making it hard. hardenable is just the ability to achieve the martensitic transformation, not how hard it can be.
4340 is a common material for cranks, I have also seen some rods made out of it as well