This is non-negotiable. Ensure you have a consistent set of backups for all Azure DevOps/TFS databases.
Security First: Managing the TFS 2015 Update 4 Patch (TFS-Update_4a-pc.zip) TFS-Update_4a-pc.zip
Preventing malicious scripts from being injected into your web portal. This is non-negotiable
Make sure you have the credentials for the TFS service accounts, as the Upgrade Wizard will likely need to be run after the file installation. How to Install Extract: Unzip the contents of TFS-Update_4a-pc.zip . Make sure you have the credentials for the
Closing loopholes that could allow unauthorized users to see sensitive project data.
TFS 2015 Update 4 has been a stable "long-tail" version for many enterprises. However, older software is a prime target for vulnerabilities. Microsoft has released several patches—often referred to in documentation as or Update 4.2 —specifically to fix:
Check the official Microsoft DevOps Blog for the latest patch notes and security advisories. Team Foundation Server 2015 Update 4 Release Notes