Back in the fifties, promotion and relegation between the top two divisions was two up, two down. Two were relegated from the Second Division and replaced by one club from each of the regional Third Divisions.
It was extremely hard to get out of the third tier back then as only the champions would get promoted.
To sell the idea of all four divisions becoming national at the tail end of the fifties, it was proposed that there would be four up and four down between Divisions Three & Four as a sweetener to get the votes needed for Division Four to become a reality. It was used as a vote winner to offset clubs who were considering the greater travelling distances that would inevitably be involved. Clubs that had failed to get promoted previously despite seasons of finishing as runners-up, liked the idea and Division Four was voted in.
Promotion/relegation between the other divisions remained at two clubs until the seventies when it became three.
So in answer to your question, it is just an historical thing that goes back to the inception of Division Four
