Famitsu interview, BL4 confirmed to run on UE5

https://www.famitsu.com/article/202412/27107

We’ve focused on making sure players encounter a variety of dynamic content as they adventure through this world. Even if you venture off the main path, there are plenty of challenges similar to what you saw in Borderlands 3. Thanks to technologies like Unreal Engine 5, we’ve been able to design each area by hand, as Randy mentioned earlier, even within a world of this scale. This allows us to greatly expand what players can do during their exploration, offering a wide range of events and activities to take on.

The interview talks about more stuff like movement options, the villain, the seamless world and the skill tree design.