I've got an existing VM with 3 VHDX files and running on a Windows Server 2012 R2 Hyper-V host.  The VM is already configure with "Hyper-V Replica", and has been replicating successfully to a DR Site host for several months now.  I've recently added a new VHDX to the VM of source server.  This new virtual hard drive (4th VHDX) is not being replicated with the replica server VM.  How can I add this VHDX file for the replication without disturbing existing settings?

In addition to that, As I checked the other way, I have removed replication from both sides and re-enabled the replication as a new with "Use an existing virtual machine on the Replica server as the initial copy" option. But there is an error while finishing it.

Your best bet for an accurate answer is to repost in the Hyper-V forum here:

https://social.technet.microsoft.com/Forums/en-US/winserverhyperv/threads

Dear Jain

I could not find the solution from the mentioned link.

It's not a solution, it's the correct forum.  This forum is not for Hyper-V, but the product Virtual Server 2005, which is not related to Hyper-V.

If you want help from Hyper-V users, the Hyper-V forum is where to repost your question.