You may notice that ASME has released PTC 4-2008 (and later revisions) which supersedes PTC 4.1 in a technical sense. Why, then, is "ASME PTC 4.1.pdf" still the most searched term? Because legacy plants, older contracts, and many university curricula still rely on the 4.1 structure. Furthermore, the 2008 version simplified many calculations, but engineers often prefer the granular detail of 4.1 for troubleshooting individual heat losses.
To obtain a legitimate copy:
Downloading the PDF is easy. Understanding Section 5 (Calculation of Efficiency) is hard. Here are the three most common pitfalls engineers encounter. Asme Ptc 4.1.pdf