Hi Daniel,
For the DCs being used for authentication, they should be identified automatically. Run "cifs domaininfo" and make sure you see all four DCs. If not, you need to check your AD sites and services to see if you have a site configured specifically for that subnet. However your ldap is an options setting, run "options ldap" to see your settings.
These should be the two you are most interested in. To enter two servers on the ldap.servers line use options ldap.servers "server1.ad,server2.ad"
ldap.servers
ldap.servers.preferred
To the DNS entries, you can have three entries live at any one time to my knowledge, so you could add one of your new DNS servers to the top of the list and the run "dns info". Do not worry if the DNS server shows DOWN, this is very misleading. DOWN could mean that you have not had enough requests to round robin over to the second server. But if it says UP and has a recent last polled, you know you have a good connection. At that point you could remove one of your old DNS servers, add the second new one to the top and repeat the process.
There should be no restart needed for either of these operations. I have done both over 50 times and never had to restart anything.