As an important note, these updates require to have the .Net Framework 4.8 installed (you can get it from here http://go.microsoft.com/fwlink/?LinkId=2085155 using the web installer or from here https://go.microsoft.com/fwlink/?linkid=2088631 using the offline installer).
As part of these updates, the issue related to address list separation being inconsistently applied (see KB 4532747) has been fixed.
The recommendation prior to deploy the CU is to have the PowerShell Execution Policy set to Unrestricted.
As reminder, it is highly recommended to not be more than 1 Cumulative Update behind (aka Exchange 2013 CU 23, Exchange 2016 CU 14 or Exchange 2019 CU 3), especially when running in Hybrid Configuration or when using Exchange Online Archiving.
The complete details of each Cumulative Update are available here:
Exchange 2016 CU 15 can be download from your Volume License portal or from the Download website http://www.microsoft.com/download/details.aspx?familyID=0d01ba8c-d415-4bc9-b5c7-3bce431d2e3b
Exchange 2019 CU 4 can be download from your Volume License portal or from your MSDN subscription https://my.visualstudio.com/Downloads?q=Exchange Server 2019 Cumulative Update 4&pgroup=