June 20, 2023
Replying to: “Please explain seasons to me”
And a little mention about the current XP in D4!
Hi Kel!
Yes and No.
For some clarity, based on my experience with Diablo 3, your current characters and game progress you build up in Diablo 4 on the Eternal Realm remain unaffected regardless of D4 season game play. You don’t have to create a new character and play any D4 season, but you will miss out on some Diablo 4 epic game goodies, loot, achievements etc… so you may want to try out season game play in Diablo 4. 
However, if you participate in any Season of Diablo 4, you need to start from level 1 to max level for every new season, etc., same as Diablo 3. It has always been this way. It’s a choice all players have in Diablo 3 and 4. So again, your current progress with your existing characters on the Eternal Realm server will not be affected in any way during any Diablo 4 season.
The first Season of Diablo 4 won’t launch until late July or latest August 2023, (no set date yet), but once you complete the main campaign story line with Lilith and Inarius you should have access to create a new seasonal character, which is separate from your existing characters on the Eternal Realm. Once the season finishes (usually in 3 months), your seasonal character(s) will be moved to the Eternal Realm where your current characters are, which you can also continue playing them.
Any loot, gold, achievements, etc. will also be sent to the Eternal Realm where your main account characters are. I’m not sure how it will work out if all of your character slots are full, though. Unless Blizzard has it figured it already.
So there is your answer: Yes, you have to recreate, rebirth, and re-level a new character for every new Season of Diablo 4.
And No, your existing characters are not affected in any way regardless of any D4 Season in the game. So you are good. 
On Players XP and D4 Improvements From Blizzard:
Lastly, each new Diablo 4 Season can also bring quality-of-life improvements, and hopefully a nice BOOST to our awful XP in Diablo 4! - We are all hoping! - You LISTENING Blizzard D4 Team!!? Stop Nerfing everything! Make the game fun and rewarding, cause it’s a hell-grind right now! - Thank you! 
I hope this post helps answer your questions on Diablo 4 Seasons and your existing characters.
Cheers,
JJ