https://us.diablo3.com/en/blog/23529004
As seen here
Kanai’s Cube: Additionally, the Kanai’s Cube will have a fourth slot this season. This fourth slot will have the choices of the first 3 slots but will not stack with any of them. The intent is to provide players with creative flexibility to their character builds.
Is that only for the season, or does it become permanent post-season?
It better become permanent!
Just as season 20 theme, which allowed you to use choose the legendary powers with no restriction of item type (weapon/armor/jewelry), didn’t become permanent, I believe the same can be said for this upcoming theme.
Oh… cool patch notes…
Might have to play Wizard again! 
It’s only for seasonal play. I wish they would make it permanent though. It would open up so many builds for non-season players
All it will be is another slot to increase your dps to overcome the overinflated health of the monsters as GRs go up. The constant increase of monster health at 17% is too high and locks you in to dealing as much damage as possible while having the bare minimum amount of toughness to survive. This gives zero incentive for any creativity with builds still.
I want to see monster health and damage addressed.
Let’s see how many classes will use F&R while slotting a ring in the extra slot just for more damage.
So gooooooooood!!! I had suggested this for so long!!!
Numerous people have suggested this. If you didn’t expect this to happen, you live under a rock and are extremely sheltered.
It is not because we had suggested it that it meant that it was going to happen… this is huge! Finally they listened to us 
Correct. At this time it is intended to be a Season only buff. Just like the previous season where we had a modified Cube.
They didn’t listen to you or anyone else with regards to this. This was going to happen.
Just like the RoRG season was going to happen. A simple bonus is an easy season bonus. I am still waiting on the season for CoE and another for F&R.
Oh wait, they basically just made this those two seasons in one almost.
Why not have the ability to take a random set property and put it in a slot that is permanent?
For instance, slotting Cain’s set bonus in a slot and benefitting from it, or Aughild’s, F&R, LoN, IB and so on.
What about doubling buff durations? Doubling CDR? Do something with the stats to where you can change up your build some. Give a buff to a stat and cap it for the season so you can’t overdo the stat.
if you want to think it was destiny, I am fine with it I am just so happy!
We need to fight and make sure the 4th slot stays permanent. It is build defining, good for the game, and opens up a lot of new/unique playstyles.
If this was their intention all along when they had the any items cube season, then I approve.
+1
IT will definitely bring new builds
Really excited what builds can work now with the 4th slot. Some builds lacked damage or toughness and this might fix them to be playable.
It is another ring slot for the majority of classes. It is either CoE or you will use F&R and slot a ring in it for most classes.
Not just a ring slot, but can be weapon or amour
It didn’t the first time. It won’t the second time. Nvm that they won’t carry over to non-season.
What he’s saying is most will end up using it as another ring slot so you can run CoE in builds that couldn’t slot it before (like hybrid builds that used CC3 and gave up CoE for RRoG) or wear F&R for the nice damage buff while still getting 2 ring slots that you previously needed to make the build work.
Things like Furnace may end up being worth it again though, so it will be interesting to see what happens.
Since FoT is such a pain to roll it might be a way to get its leg bonus while wearing HF or Travelers Pledge before you find a usable FoT.
LOL… well thats true
ah ok. Misunderstood him. Well, in that case POG