My gut feeling is "no", but an Oracle salesperson told me yes. I am not talking kernel zones here btw.
No, it only supports Solaris 10 branded zones. Solaris 11.3 branded zones shouldn't be needed as 11.4 is a minor release, and application compatibility should be maintained for most applications.
Note that it is possible to run Solaris 11.3 in a kernel zone on Solaris 11.4, but that's full virtualization of a separate kernel, not a shared kernel as you get with branded zones.