Billy from Epic Games has in this video given a brief look at Fortnite: Save the World (Cooperation mode) in the latest dev video update. Below is a brief summary.

As for energy ammo, it will increase to 25% more ammo per craft, and 20% more ammo per container in the world.

Patch 4.2 will introduce perk-re-rolls, and it seems like you can reroll individual weapon perks. He mentions it will not cost V-Bucks, but instead resources found within the game. You will be able to choose yourself which slot you want to reroll. Though, he mentions that not all of the slots will be rerollable, and some will have different options.

For Patch 4.0, a new event, a military weapons set and “shadow agent heroes” are planned and Billy promises for more updates the following week.

You can see the video  yourself below.