Samba 4.4 is set to bring asynchronous flush requests, various new sub-commands, improvements to Samba KCC, Active Directory improvements, CTDB changes, and more. The async flush requests are described via the what's new documentation as "Flush requests from SMB2/3 clients are handled asynchronously and do not block the processing of other requests. Note that 'strict sync' has to be set to 'yes' for Samba to honor flush requests from SMB clients."
Samba 4.4 RC1 is tagged with Samba 4.5-pre1 now being the latest Git code. I haven't yet seen an official Samba 4.4 RC1 announcement go out but for now you can fetch the code via Git. The release plan currently has the Samba 4.4.0 release scheduled to happen for early March.