Does Garmin have any plans to add in the Smartwake feature to its watches anytime soon (particularly the Venu or Vivoactive 4s)?

I don't think you can expect ANY new features on the Vivoactive 4/4S. The watch has been on the market for a while, and will probably be replaced soon. Maybe a Vivoactive 5?
The Venu 2 is quite new, and can perhaps still get features.

Ovekvam is right, I doubt that VA4 would get any new bells and whistles.

And just so you know, I was Garmin user five years ago moving to it from Jawbone, I was surprised to find out that my Vivosmart HR doesn't have  smart feature which my Jawbone had(not that it was working flawlessly there, but at least sleeping was more reliable).

So since then I submitted requests to them via https://www.garmin.com/en-US/forms/ideas/ talked via mails trying to make them fix their poor sleep tracking and add that smart alarm feature too.

So far as we can see no new generation of watches has it, in time when competitors all got that feature. Now seeing how Garmin bought out FirstBeat they can probably add it even easier then before but they need to have a motivation for that. I guess it will be a feature for some new generation of devices to make people upgrade from their old watches at some point.

Anyway Garmin is probably listening to their users, because back then my smart band had a lot of bugs with overwriting exercises and few other glitches, and after I took my band out of the box and got new firmware update I was surprised to see that they were addressed mostly. Too bad they failed to fix any during three years of my active use of the device. So it's still worth to report bugs and submit ideas, in my opinion. If something get a lot of supporters Garmin can possibly implement it at some point.

I'll be celebrating if they at least fix bugs!!