Hi Pete,
I depends upon what is in the pipe.
B31.4 is "Pipeline Transportation Systems for Liquid Hydrocarbons, and other Liquids". There is no comprehensive document within the Code of Federal Regulations that addresses this (LIQUID) type of piping.
B31.8 is "Gas Transportation and Distribution Piping Systems". These systems come under (are regulated by) 49 CFR 192 (i.e., Title 49, Code of Federal Regulations, Part 192), "Transportation of Natural and Other Gas by Pipelines: Minimal Federal Safety Standards". Virtually every aspect of GAS piping degign for cross country transportation is regulated by 49 CFR 192. ASME also publishes a "guide Document" from within the B31 organization for understanding the CFR.
I hope this helps.
Best regards, John.