Season 3 for Diablo 3 will be wrapping up later this month. Today Blizzard announced the exact time it will end along with the start date for Season 4.
Blizzard said on their website that Season 3 will conclude in a couple weeks. Here's the exact time for each territory:
Season 3 players are also able to earn a special portrait frame and pennant by racking up achievements. They'll unlock the season-exclusive pennant by hitting 100 Achievement points in the Season and a portrait frame by earning 400 points. Both will be available through the Wardrobe located next to the Mystic.
The heroes you created for Season 3 will be be lumped into your pool of Normal or Hardcore non-Seasonal heroes. They'll keep all of their gold, Paragon experience, items, Blood Shards, artisan progress, and shared stash purchases.
There are some caveats here, though. If you have items in your Seasonal stash, they won't transfer over to your normal stash. Instead, they'll be sent through the in-game mail system to your non-Seasonal characters. Once you log into the game following Season 3's end, you'll have 30 days to claim these items.
If your pool of non-Seasonal characters are at the Blood Shard cap, the Shards you earned in Season 3 can put your over the cap. You can spend them normally but can't earn any more until you've fallen below the cap.
Achievements unlocked through the Season will be applied to your wider account. Conquests won't be but you can look back at these accomplishments in your Records page.
Blizzard hasn't announced any specifics for Season 4's rewards. If past Seasons are any indication, players will have a chance to find exclusive Legendaries during their travels. I imagine players can earn two more transmog items as well.
Season 4 is bound to be very different than past Seasons thanks to Patch 2.3. The future update will make several changes to Adventure Mode along with a powerful new relic.