According to SAE J404 Chemical Compositions of SAE Alloy Steels, alloy 4140 has an allowable carbon concentration (in mass percent) of 0.38-0.43 while alloy 4142 has an allowable carbon concentration of 0.40-0.45.
According to SAE J1268 Hardenability Bands for Carbon and Alloy H Steels, alloy 4142 has better hardenability.
4142 is not a prehardened version of 4140. They both require the same thermomechanical processing to develop their properties.