Did you ever formally define a 4D or 5D critter?
If by this question you ask were they a specific race, then yes mine were the "Eldren". They were typically 4D beings, and kind of immortal galactic rulers, and somewhat hostile if not indifferent to the mains.
They could slip out of the 3D ("plane" if you will) and observe from outside. It was impossible to withhold secrets as they could necessarily see anything they choose to look at such as the inside of a safe. No walls could stop them should they choose to introduce themselves right into the heart of your fortress. They could also duplicate themselves, if you can picture a 3D person folding a 2D space so he intersects it twice, thus appearing in two different places to a 2D creature.
To counteract their power, I made gravity harmful to them and upped their energy requirements so they need hot stars and will gradually freeze in the deep orbits. This essentially keeps them off Earth-like worlds, giving my cast some breathing room
