Because of the missing MIT Kerberos support in Samba, you can not use the Samba packages provided by Red Hat Enterprise Linux 7 as a Samba AD DC. As alternative:
- build Samba. For details, see Build Samba from Source.
- use 3rd-party packages with AD support from a trusted source.
Try reading it, this confirms what you fear, you cannot have a samba4 DC with the official samba packages, so you are going to have to compile it yourself, but this might be easier said than done due to the kerberos problem, might just be easier to install Debian on the server
Fedora and RHEL are using MIT Kerberos implementation as its Kerberos infrastructure of choice.
While the Samba Active Directory Domain Controller implementation is the Heimdal and so not available with MIT Kereberos at the moment.
Only way to have full Samba4 functionality on RHEL now, is to compile from source.
I'm not surprised the RH guys dont want to change Kerberos implementation because of Samba, as it is interwoven with many other OS related things, but it would surely help, if RH would maybe said something about ETA? Or progress?