the daizenshuus clearly state these 4 "galaxies" were created for convenience to give the kaiohs a more clear area to rule over
they aren't used in the way we understand the term "galaxy" in our universe, in those instances. there are several other things from the books that liken the universe to our own..here's a few;
The Universe:
The endless, expansive space wrapped around every celestial body that exists in the Living World. Along with the "Demon Realm," it is a single, large world in the Living World. In the world of Dragon Ball, the universe is located beneath the World Beyond, and it is hermetically sealed by a barrier that is engraved with a strange design. Within the universe, there are "nebulae" composed of innumerable stars, and there are "galaxies" composed of innumerable nebulae.(...)
The Universe Õ«çÕ«Ö/Uchuu Translation: the universe, space, outer space
The universe is the upper portion of the living world, and is the realm where Goku and the other main characters live and die. It's the only portion of the DB world that even remotely resembles reality, being based off of the actual universe (or at least the universe as seen in countless sci-fi space operas like Star Wars). As such it consists of countless planets, stars, and other celestial bodies.
Úèǵ▓│ Galaxy
A local collection of stars and planets in outer space. A nebula is a collection of stars and planets, and in turn a collection of several nebulae is called a galaxy. Being responsible for the north/south/east/west, the four Kaios in practice oversee these galaxies. The divides between north/south/east/west galaxies are units which the celestial gods and Kaios produced for occupational purposes, in order to unify the galaxies which exist infinitely in outer space.
(...)The various Kaio govern the infinitely expansive universe. This is primarily determined by the compass points, while the Dai Kaio serves as general manager of the places governed by the Kaio, and beyond that the Kaioshin of the North, East, South, and West govern the universe. Ultimately, the Dai Kaioshin supervises the universe in its entirety.
Here's the easy way to explain this; imagine our own universe. a group of gods exists unbeknownst to us, and has divided it in 4 parts to make it convenient for them. we still have our own innumerable galaxies as we perceive, but the gods have divided the universe in 4 parts.