So, you’re interested in non-euclidean level design, eh? Me too, but… Well there’s a bit of bad news. I had spent roughly 3 weeks researching the topic so I’ll link you to what I’ve found so far, but UE4 doesn’t naively support this feature. It’s all just workarounds and smoke and mirrors. In order to get truly seamless portal doors you would need to implement C++ like what they did in Antichamber.
If you’ve got closed environments it’s not so hard, you can use level streaming to bring levels in and out between shut doors. But if you want a room that’s bigger on the inside, that’s a bit more of a problem. Anyhow, hope these help you get started. If you find out how to achieve it let me know.