And from the midst of it came the likeness of four living creatures. And this was their appearance: they had a human likeness but each had four faces, and each of them had four wings. Their legs were straight and the soles of their feet were like the sole of a calf's foot. And they sparkled like burnished bronze. Under their wings on their four sides they had human hands. And the four had their faces and their wings thus: their wings touched one another. Each one of them went straight forward, without turning as they went.Effectively, beings that could move in the fourth dimension would be nigh-unstoppable by any mere adventurer, capable of ignoring the adventurer's armor and weapons at will. Of course, if the monster could attack the adventurer, at least some of the monster has to be on the same phase(?) as the adventurer.
Mechanically, this could be represented by a damage cap for four dimensional foes, such as five points. This damage cap would apply to all attacks (even spells) to represent how it was because the body was on a separate phase not just because it was resistant. Of course, on their turn, the four dimensional foe could shift to a phase where the party couldn't reach them at all, but they couldn't hurt the party either.