Unfortunately, no. I guess it is because Solaris 11.4 was originally going to be Solaris 12. It has a lot of new stuff and modifications compared to 11.3. I guess they wanted to optimize it for the newer SPARC processors (or get enterprises to update their old systems).
T4, FujiSPARC-X/X+/XII, or AMD64/EM64T.
Outside of this announcement, I think all of the releases are available to download for noncommercial evaluation and development.
Yes, but you don’t get access to the update repositories which is what you want to keep the system secure.
I don’t think Solaris gets much attention from either the blue or red side these days. Patched or not, I would keep a Solaris machine segmented off with my other retro crap and not use it for anything public-facing where it might be exposed to attack.