User1 and User4 have the necessary permissions to delegate the addition of workbooks to the Microsoft Sentinel workspace. User1, as the Owner of the resource group RG1, has wide-ranging permissions that include the ability to add workbooks to the Sentinel workspace. User4, being assigned the Microsoft Sentinel Reader and Workbook Contributor roles, can view the Sentinel workspace and contribute to workbooks, respectively [1] [4] [5]. User2 and User3, however, do not have the required permissions for this task [4] [5]. Therefore, the answer is: User1 and User4 only.