If 1 is numerology's number for starting and 3 is its number for expressing, 4 is where the tradition puts the part of a project nobody photographs — the framing, the scheduling, the follow-through that makes the first two mean anything.
The symbolism leans on geometry again: four points are what it takes to build a stable, closed shape on a flat plane — a square, the simplest shape that can't wobble the way a triangle can be tipped. Numerology writers use that stability as a direct metaphor for the number's traditional personality theme.
Vedic-influenced numerology sometimes assigns 4 to Rahu, a shadow point in Vedic astrology associated with disruption and unconventional paths — a notably different emphasis from the "steady builder" reading common in Western practice, and a useful reminder that even a single digit doesn't get one universal story.