The manganese allows for heat threatment to a higher tensile strength without getting brittle.

4130 is meaningless, because in normal state it is no better than a good structural steel. The tensile strength after heat threatment is what makes the difference. And that can vary greatly depending on the process employed. The same piece of C'moly can be made to exhibit totally different tensile strenghts and characteristics depending on the heat threatment.

again 4130 is the alloy, after that, it is meaning less