composer/tests/Composer/Test/Installer/Fixtures/installer-v4/composer.json

{
    "name": "",
    "version": "4.0.0",
    "type": "composer-installer",
    "autoload": { "psr-0": { "Installer": "" } },
    "extra": {
        "class": [
            "Installer\\Custom1",
            "Installer\\Custom2"
        ]
    }
}