The seasonal mechanic is on the lighter side this time because of the massive overhaul they did to itemization. Essentially it comes down to a reputation grind for rewards, and a few minor things.
Seasonal themes are never in the patch notes, they have their own section.
Avalon also made a great post:
Feel free to play on the Eternal Realm as some are opting to do though, or wait until S5 as others are planning to do. Up to you.
Omitting because apparently I offended the OP, my bad.