ISO 15000-2, the ISO-approved version of AS4 is mentioned in COMMISSION IMPLEMENTING REGULATION C/2022/5628, EU legislation for the so-called technical system for the cross-border automated exchange of evidence and application of the "once-only" principle. This regulation was published last week:
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=PI_COM%3AC%282022%295628 This system is expected become one of the largest European cross-border public services systems. It is set up to support the EU/EEA internal market. More technical information on this system should come out in the near future, including on other standards it uses.