Royceb,
From all I've read in the Samba4 traps and done with Zentyal since the early v 3.0 releases, there is no difference between a Domain Controller (DC) and Additional Domain Controllers (ADC), EXCEPT that the DC is the only machine that was originally involved in provisioning or establishing the domain and all ADCs have been estabished by replication from the DC.
Although Zentyal will forever show this distinction in the Domain -> Settings page of its web-GUI, I suppose - to answer your question - that the machine holding the PDC Emulator FSMO Role is the "pdc". If you wish to promote an ADC to DC, move all FSMO Roles accross to the new machine. This could be done with "samba-tool domain dcpromo ..." (this never worked for me) or with "samba-tool fsmo transfer ... | seize ..." (has worked for me).