Thank you for the response. These are the steps I followed to delete the older nodes
1. added node 3 and node 4 to the cluster
2. made node 3 as primary
3. disabled replicaction for node 1 and 2
4. removed node 1 and node 2 from cluster
please let me know if this approach has any issues.
Also, what could be the reason for the objects count mismatch?