For AISI 4140 versus AISI 4142 the only main difference is the carbon content, and it's pretty small. The balance of the chemistry is the same. The elemental compositions can be seen here:
Carbon 0.38 – 0.43 for 4140 versus 0.40 – 0.45 for 4142
Manganese 0.75 – 1.00
Phosphorous 0.035 Max
Sulfur 0.040 Max
Silicon 0.15 – 0.30
Chromium 0.80 – 1.10
Molybdenum 0.15 – 0.25
Maui