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No doubt about what you stated above especially if the material is supplied in a quenched and tempered heat treatment condition. However, pressure vessels used in military or space applications do not equate to pressure vessels that are designed for public use. In the US, pressure vessels that are rated for 15 psig or higher pressure must be built to ASME B&PV Code or an accepted pressure vessel code, this is the law.
At 0.40 w/%C, notch toughness is marginal in low alloy steel unless it is supplied in a quenched and tempered condition. Also, hardenability of 4130 and 4140 is limited in assuring a uniform quenched and tempered microstructure for low temperature toughness. When you have quenched and tempered heat treatment, additional welding controls come into play to prevent untempered martensite in the base metal HAZ, and to prevent over tempering to reduce strength. You also have temper embrittlement issues to deal with these materials. ASME Boiler and Pressure Vessel Code is conservative and trying to fabricate pressure vessels out of 4130, 4140 materials are not favorable.
ASME B&PV Code deals with all sorts of pressure vessels that are designed based on past experience and to assure public safety. Using lower alloy grades of steel with a fine grained structure and a N&T heat treatment is much easier to work with, especially regarding weldability because of the lower carbon content (less than 0.35%). Also, temper embrittlement is not a concern with most pressure vessel quality steel plates.
