That should work. Assuming I understand correctly. And what I understand is that you have
1- replica-set-1where all the members are running on k8s-cluster-1
2- you want to add member-4 to replica-set-1 that runs on k8s-cluster-2
3- make member-4 primary once it has synced
4- 2 other members to replica-set-1 also running on k8s-cluster-2
5- finally, remove all members from replica-set-1 that runs on k8s-cluster-1
That should work on the mongo side. The other thing is how the application connects to your replica set. If you are using SRV and update all the DNS information correctly it should be fine. If it connects using member addresses of k8s-cluster-1, you have to update the connection string to include members of k8s-cluster-2.