Both 4130 and 8740 chrome moly are alloy steels with similar chemistry. The 4130 has only .3% carbon and can’t be hardened as high as 8740, which has .4% carbon. Also, 8740 has about .45% nickel and 4130 has none. Both have moly (most alloy steels have moly). The chromium content of 4130 is slightly higher, .95% instead of .55%. However, 8740 is generally considered to have slightly better toughness due to the nickel.
