Solaris 11.4 has resumable send stream enabled by default. If you pass '-s nocheck' to the 11.4 'zfs send' then it will remove the feature flag and metadata for resumable send and 11.3 should be able to see it.
From the zfs(8) man page on 11.4:
-s check Specifies that the output stream will use a send stream format that uses per-record checksums. This format is the system-wide default. When an interrupted transfer is resumed by using a receive checkpoint (see zfs send --C), the use of the 'check' format before the outage enables the resumed transfer to pre- serve and reuse the portions of the interrupted snapshot that were already successfully received. -s nocheck Specifies that the output stream will use a legacy send stream format without per-record checksums. This format is suitable for transmission to older systems that do not support the new format.
$ zfs send SomePool1/SomeThing@123 | zstreamdump -v | grep 'features ='
features = a4
Note that a4 is the flags that were in the send stream:
$ zfs send -s nocheck SomePool1/SomeThing@123 | zstreamdump -v | grep 'features ='
features = 24
Darren J Moffat
Senior Software Architect - Oracle Solaris Engineering
That was it.