From SAE J404 Chemical Compositions of SAE Alloy Steels and SAE J1268 Hardenability Bands for Carbon and Alloy H Steels:

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

Does the spec call for required surface hardness or hardness profile?

This suggests 4142 HT may be a somewhat standard product for making shafts and whatnot.
http://www.speedymetals.com/information/Material63...
4142 HT Turned, Ground and Polished is a chromium-molybdenun alloy steel that has been hardened to 27 to 32 Rockwell C and is ready to use while still machinable and of exceedingly high strength.
MECHANICAL PROPERTIES
Tensile Strength (PSI) 140,000
Yield Strength (PSI) 90,000
Reduction of Area 45
Elongation in 2" 20
Brinell Hardness 300
The above values are average and may be considered as representative of 4140 HT TGP

Note the specs have a comment referencing 4140, not 4142. No clue where the typo is.
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https://www.lindquiststeels.com/documentation/4140...

•4140 HT – prehardened condition which is heat treated to a medium hardness (HRC 28/32). 4140 HT is supplied in many forms such as; hot rolled or turned, ground and polished rounds and DCF plate which is used as a holder material for high strength structural components.

Many mills will dual certify their 4140 as 4140/42, meaning it meets the chemistry requirements of both 4140 and 4142. The same would be true as 4140/42HT.

See if your suppliers can find some dual certified material.

I have ordered the dual 4140/4142 HR HT