Because the password is not universal, you must locate it physically on the server hardware. Look for a (often called a "toe tag"):

iLO 4 (Integrated Lights-Out 4) is a remote management interface for HP ProLiant servers. It allows administrators to remotely access and manage the server, even if the operating system is not installed or not functioning.

Re: Can not login with default username and password (from sticker) in iLO4. You can log in to iLO configuration utility by pressi... Hewlett Packard Enterprise Community HPE Integrated Lights Out (iLO 4) - Managing local user accounts ... Adding user accounts (iLO 4 Configuration Utility) * Optional: If user access the server remotely, start an iLO remote console ses... Hewlett Packard Enterprise Recovering the iLO default password using the iLO web interface * Removing the system board. * Installing the system board. * Binding the DC-SCM with the HPM. Binding the DC-SCM with the HPM usi... Hewlett Packard Enterprise Logging in to the iLO web interface - HPE Support Enter a directory or local account login name and password, and then click Log In. If iLO is configured for Kerberos network authe... Hewlett Packard Enterprise

Unlike modern iLO 5 or iLO 6 devices that force you to change the password at first boot, :

But for iLO 4? No tag. No unique secret. Just Administrator and an empty field.

But before you can do any of that, you need to get past the login screen. And this is where many IT horror stories begin.

Here is the complete text regarding the default login credentials for HP iLO 4.