Which domain controller must be online to meet the technical requirements for DC4?

DC1 must be online because it holds the Schema Master FSMO role. When promoting a new domain controller (DC4 in this case) to an existing forest, the Schema Master must be available to update the Active Directory schema with the necessary attributes and classes for the new DC. Without the Schema Master, the promotion process cannot proceed. DC2 and DC3 are likely Global Catalog servers or hold other FSMO roles (like RID Master or PDC Emulator), which are not strictly required for the initial promotion of a new domain controller, although they are critical for ongoing operations.