If I understand correctly the question, then when you connect the OpenVPN client to the server, the server only gives out 2 dns server addresses, and you need 4.
If so, then in the config file of the OpenVPN server you need to add these dns.
the configuration file is usually /etc/oepnvpn/server.conf
push "dhcp-option DNS 192.168.50.10"
push "dhcp-option DNS 192.168.70.10"link to howto