The first 4th Dimension appeared in Ultraman. It was much different than the one in Ultraseven as the two series were supposed to set in different universes at the time. Ultraman's 4th Dimension is a warped universe. Stairs lead to nowhere, jumping into a wall will cause teleportation and surroundings may become distorted. The second Fourth Dimension is a place where normal components of the Third Dimension are not physically possible. Dan Moroboshi tried to use his Ultra Eye and Capsule Kaiju to no effect as the different dimension prevents this from occurring. The dimension also serves as a stop for Alien Icarus' flying saucer after its attacks on humanity. The Fourth Dimension is connected to the Third Dimension by a control device. If destroyed, the two dimensions will no longer be connected.
In Ultraman (1966), Bullton can teleport people into the 4th Dimension and the rock most likely originated from there. The major inhabitants of this dimension are the Alien Icarus. It is unknown if there are other aliens or monsters in this dimension. The Icarus has the technology to let them cross to Earth from the 4th dimension.