Sonos hasn’t announced anything.

Is the Gen 3 not sufficient for you?

I have the Arc and 2 Fives for my home theater setup. I am considering adding the Sub Gen 3. However, if a Sub Gen 4 is going to be released within 12 to 18 months I would rather wait for it.

Additionally, I am considering replacing my 2 Fives with Era 300s in my home theater setup. I have watched countless YouTube videos, and read many posts on Sonos.com. I am having a hard time discerning if that would be an improvement to my current setup. The Era 300s' front firing drivers being turned off in this setup causes hesitation for me. I have read pros and cons related to that subject. Any input would be appreciated. 

Sonos doesn’t announce new hardware in that time frame, usually we get weeks of notice, not months, before the new product goes on sale. So no one except Sonos knows, and Sonos won’t say. 
 

As to replacing the Sonos Fives with Sonos Era 300s, it depends on several factors, in my opinion. How much content that you watch on your TV is in Atmos? And the second is subjective, how your ears and brain perceive the difference. My suggestion would be to make the change by purchasing from Sonos, and return them if you don’t like them.