If your first server is still active you should change your name server IP addresses after the transfer is done.This will prevent Server 2 for showing errors to clients that are still not transferred to server 2.Also you don't need ns3 and ns4 if your first server is going to be inactive.Simply update the ns1 and ns2 IPs once the transfer is done.
Failing to do so,your clients domain names will still be pointing to ns1 and ns2 name servers ( server 1 )
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