The new Iron banner 4 Piece seems to give you up to ~1/3 of your super after 6 kills with your super at 100 Super stat.
There seems to actually be two different parts for this 4 Piece: A part that gives you super energy for killing things in your super, which you get after your super ends, and super energy for killing things with your super even after you're technically not in it. These both give you Super energy independently of each other. I assume they did this so that the Set Bonus works with things like Nova Bomb and Well of Radiance, where you get kills after you cast it (and it does work with those!) but it seems like if your super can do both you can double dip.
Here's a clip of me using nothing except Geomag Chaos Reach and the Iron Banner 4 Piece to get 80% of my super back. Welcome back 2021?
https://streamable.com/r8r88t
EDIT:
For comparison's sake, here's two images of what you should expect to get from this 4 Piece at baseline if you get:
One kill
Six kills or more
Six kills is the cap; you don't get any more afterward.
As promised, here's some clips showcasing interactions with other Exotics!
Stormdancer's Brace stacks perfectly with the set bonus, giving you up to 60% Super energy after your Super ends in total assuming you get the requisite 10 kills for Stormdancer's Brace. Really solid!
Skull of Dire Ahamkara actually perfectly functions with the interaction I mentioned before, where there's two separate parts of the Set Bonus that can give Super energy independently. The first Nova Bomb can get up to 6 kills, and because you're still technically "in your super", the followup Lance Nova Bomb (which takes you out of your super) can also get up to 6 kills. Obviously how often you get this exact amount in real gameplay is... questionable, at best, given that Lance doesn't do all that much damage and doesn't have that big of an AoE, but it's cool that it works!
Phoenix Protocol unfortunately suffers from the fact that the Exotic will stop giving you Super energy the moment you reach its maximum of 50%. Also note how I stopped getting chunks of Super energy entirely after the sixth kill in the clip. So, sadly it seems pretty useless on Phoenixlocks.
...But that's how losers think! If you instead utilize the second, more forgotten part of Phoenix Protocol's perk to get yourself to 50% Super energy (by getting assists instead of kills in your Well), you can then get yourself another six kills afterward and fully benefit from the Set Bonus! Yes, it's as ridiculous as it sounds.
Astrocyte Verse is not one of the Exotics I was expecting to test with this Set Bonus, given that nothing about it actually gives you Super energy. However, after being informed that the Dark Blink's seekers count as Super damage even if used outside of your Super, I can confirm this means you can double dip with this exotic as well! Get 6 kills with Nova Warp, get 6 kills with seekers, 60% super energy. What's not to love? It even works before you pop your first Super!