The Fourth Dimension, also known as the Space Between the Worlds or simply the Space Between, is that world that exists in the void outside of all the dimensions. Anyone travelling between worlds, between times, or even between life and death must pass through this place.
This is also the location of the Time, World, and Spirit Gates.

The Gates
The Gates of this World were created as physical representations of otherwise intangible and limitless boundaries – those of Time, Space, and the Spirit. In an effort to more effectively regulate, control and maintain the balance of this Universe, each Gate functions as a focal point or nexus for its respective domain, with its dedicated Guardian to protect it. Each of the Gates is tied fundamentally to the Galaxy Cauldron, which is in itself a sort of Gate, though mainly one-way. The Cauldron is a nexus of Creation, where new Starseeds are created, new souls are born, and old souls are regenerated. The Spirit Gate supplies experienced souls for the Cauldron, the World Gate gives access to souls from other Worlds, and from the Time Gate one can intercept the transfer of souls and Seeds at any point in time.

The World Gate

Considered by some to be of less import than the Time and Spirit Gates, the World Gate was created to be a nexus for interdimensional travellers, meant to funnel those passing into and out of this World to a specific and easily defensible location. Other scattered means of entry and exit still exist, however, and the duties of the World Guardian include finding and sealing these thin places between the Worlds. The World Gate primarily links those Worlds in which Senshi can and do exist, and is the only secure means of travel between those Worlds.

The Spirit Gate

Due to its direct connection with the Galaxy Cauldron, the Spirit Gate is often considered to be the most important of the three Gates. Since the spirits of the dead - good, evil, weak, and powerful – all pass the Gate prior to being processed in the Cauldron, it becomes a point of interest to those with nefarious as well as benign intentions. Thus the Spirit Gate has seen several major battles for control.

The Guardians

The only beings who live in the Fourth Dimension are the Guardians of the Gates; the Time Guardian, SailorPluto, the World Guardian, SailorDís, and the Spirit Guardian, Hades.

While they possess the powers of Senshi, Guardians fill a more specific role. Guardians can be and sometimes are the chosen Senshi of their respective systems or planets, but are usually culled from a star system that already has protectors, or from a planet that has been destroyed or otherwise lost. Guardians represent and uphold Order in the Universe, and once gifted with their title and specific powers over their domains, are charged to remain at their posts at all costs, unless absence is unavoidable. Guardianship, with all of its responsibilities and dangers, also affords a certain measure of immortality. The three Guardians of the Gates of this World spend the majority of their time in the Space Between the Worlds, and thus suffer less from the passage of time and the effects of Entropy than ordinary Senshi or civilians do. Guardianship can be passed on through family lines, but sometimes the title is bestowed upon or earned by entirely unrelated individuals.