Hi, looks like 15.4.x has broken softwareupdate --verbose --fetch-full-installer --full-installer-version for all versions (still works in 15.3.1)
softwareupdate --verbose --list-full-installers
Finding available software
Software Update found the following full installers:
* Title: macOS Sequoia, Version: 15.4.1, Size: 15244333KiB, Build: 24E263, Deferred: NO
* Title: macOS Sequoia, Version: 15.4, Size: 15243957KiB, Build: 24E248, Deferred: NO
* Title: macOS Sequoia, Version: 15.3.2, Size: 14890483KiB, Build: 24D81, Deferred: NO
* Title: macOS Sequoia, Version: 15.3.1, Size: 14891477KiB, Build: 24D70, Deferred: NO
* Title: macOS Sequoia, Version: 15.3, Size: 14889290KiB, Build: 24D60, Deferred: NO
* Title: macOS Sequoia, Version: 15.2, Size: 14921025KiB, Build: 24C101, Deferred: NO
* Title: macOS Sonoma, Version: 14.7.5, Size: 13337289KiB, Build: 23H527, Deferred: NO
* Title: macOS Sonoma, Version: 14.7.4, Size: 13332546KiB, Build: 23H420, Deferred: NO
* Title: macOS Sonoma, Version: 14.7.3, Size: 13334287KiB, Build: 23H417, Deferred: NO
* Title: macOS Sonoma, Version: 14.7.2, Size: 13333067KiB, Build: 23H311, Deferred: NO
* Title: macOS Ventura, Version: 13.7.5, Size: 11916960KiB, Build: 22H527, Deferred: NO
* Title: macOS Ventura, Version: 13.7.4, Size: 11915317KiB, Build: 22H420, Deferred: NO
* Title: macOS Ventura, Version: 13.7.3, Size: 11915737KiB, Build: 22H417, Deferred: NO
* Title: macOS Ventura, Version: 13.7.2, Size: 11916651KiB, Build: 22H313, Deferred: NO
* Title: macOS Monterey, Version: 12.7.4, Size: 12117810KiB, Build: 21H1123, Deferred: NO
* Title: macOS Big Sur, Version: 11.7.10, Size: 12125478KiB, Build: 20G1427, Deferred: NO
* Title: macOS Catalina, Version: 10.15.7, Size: 8055650KiB, Build: 19H15, Deferred: NO
* Title: macOS Catalina, Version: 10.15.7, Size: 8055522KiB, Build: 19H2, Deferred: NO
* Title: macOS Catalina, Version: 10.15.6, Size: 8055450KiB, Build: 19G2021, Deferred: NO
softwareupdate --verbose --fetch-full-installer --full-installer-version 15.4
Scanning for 15.4 installer
Install failed with error: Update not found