Install old Cumulative Update SQL Server 2016 Standard We were running on SQL Server 2016 Standard Edition 2016 SP2-CU4 on our ALWAYS-ON HA cluster.
Due to some recent issues one of our nodes got corrupted and we had to replace it with a new node. Since this was a fresh image from AWS Marketplace it came with SP2-CU13 (latest) out of the box.
The client wants this to be rolled-back to the original SP2-CU4 (no plans/requirement of upgrading to CU13).
As a primary step I tried to install the SP2-CU4 KB article on the instance but that does not work with the attached error message. (No SQL Server instance or feature can be updated on this computer)

Q1 - How can SP2-CU4 installed on the server.
Q2 - What is the impact if the cluster runs on different CU's.
You said you are running a new instance of SQL 2016 SP2 CU13. In order to install CU4, you will need to uninstall later CUs. I just spun up a new EC2 instance with SQL 2016 STD and went to Control Panel &gt; Programs &gt; Programs and Features &gt; Installed Updates and saw Hotfix 5820, 5698, and 5598 after SP2. They should be uninstalled in the order listed from the newest to the oldest. After that you will be on SP2 and you can install SP4.