Four as a foundation
When numerology books describe four as stable, solid or foundational, they are using an old image. People in many places have pictured the world, and the things they build in it, as resting on four supports or reaching to four limits. The image comes first. Numerology, with its habit of turning symbols into digit meanings, came much later.
As with any number belief, this is a symbolic tradition and not a verified property of the digit. The question here is why four attracted the idea of stability, and what that does and does not show.
The four quarters
The most widespread source is the compass. Anyone standing on open ground can name front, back, left and right, and from that arise four directions. Many cultures turned this into a map of the world. In Mesopotamia, rulers from the time of the Akkadian kings used the title “king of the four quarters”, meaning ruler of the whole world. The title is the same idea as universal rule, expressed through four regions.
Biblical writers use the phrase “four corners of the earth”, and it appears in both the Hebrew Bible and the Book of Revelation, where angels stand at the earth’s four corners. The geography is symbolic. The phrase says “everywhere”, and the use of four suggests a complete frame.
Other examples are scattered across the world. The Inca named their realm Tawantinsuyu, a name usually understood to mean four parts joined, with the Tawantinsuyu four parts reckoned outward from the capital, Cusco. The Navajo sacred four directions give the number a central role in ritual order. Ancient Egyptian belief placed the four sons of Horus as guardians of the organs stored in canopic jars. These are traditions with separate roots. They show that four recurs as a frame, not that any one tradition borrowed from another.
Squares, pillars and buildings
The second source is architecture. A square has four equal sides and four corners, and a square base is a natural way to lay out a stable building. Egyptian pyramids have square bases, and temples and houses of many cultures were laid out as rectangles on four walls. Our language still reflects the idea in terms like cornerstone and foundation stone.
Sky and earth appear in some cosmologies as a roof held up by supports. In some Egyptian depictions the sky rests on four pillars, though the details differ between sources. A well-known Chinese formula describes heaven as round and earth as square, with the square earth tied to four sides and four directions. In Greek philosophy the four elements doctrine, with fire, air, water and earth, divided material things into four basic parts, and Plato’s account of the cardinal virtues also numbers four of them. In each case, four supplies the sense of a complete, grounded whole.
A small caution: stability in physical objects does not actually follow from the number four. A three-legged stool will sit without wobbling on uneven ground, because three points always define a plane, while a four-legged table may rock if one leg is short. So the stability of four is a symbolic idea and not an engineering one. The symbol endured because the square and the cardinal points were easier to imagine than a triangle for a whole world.
From symbol to numerology
Pythagorean tradition fits into the same picture. Four is the first square number (2 × 2), and the Pythagoreans honoured the Pythagorean tetractys, with its four rows of points. Later writers developed an extensive square and cross symbolism from the same shapes.
Modern numerology picked up this inheritance. Writers in the early twentieth century described four as the number of foundations, order and the material world, helped along by the planetary and elemental associations they used. When they turned to personality description, “foundation” became “steady, dependable, grounded”. That last step is the work of numerology books, not of ancient sources.
Here the belief is also contested within numerology itself, as some systems associate four with disruption and the unconventional, so even the foundation reading is not universal.
What this does not show
None of this suggests that people with a four in their birth date or name are steadier than anyone else. The symbolic record shows how people have used four to describe the world, and that record is varied: where some cultures made four a number of completeness, others have avoided it for reasons based on language. A careful reader can enjoy the symbolism, which is real cultural history, and keep it separate from the claims of modern digit readings, which are beliefs without demonstrated effect.