I think you're mingling 2 concepts of the ldap server, being group membership information and userID information
the ldap profiles can be used to extract groups, users and group membership from the ldap server. in the ldap profile one server is used for retrieval and authentication, as all servers in the profile should contain the same information (the ldap tree with all the groups and users for it's domain)
userID does not use the ldap profile: user to IP mapping is gained through UserID agents, Captive portal, API calls ,.. and only after the firewall gains a user-ip mapping is the user matched to a user object and group membership obtained through the ldap
so no information should be lost unless your ldap servers do not replicate their ldap tree information
for UserID information, all 4 servers should be polled by a UserID agent to detect the distributed logins
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