Spiritual
The bull symbolizes religion, dharma, and its four legs represent the pillars that uphold it: truthfulness, purity, austerity, and mercy. Just as a table stands firmly on four legs, religion remains steady as long as these four virtues are being upheld.
In the Śrīmad-Bhāgavatam, Canto One, Chapter Seventeen, it is described how King Parīkṣit encounters a wounded bull with three broken legs and a cow weeping beside him. The bull is dharma personified, and in that scene, he is being attacked by Kali—the personification of the present age of quarrel, in which spiritual values are gradually declining. This image powerfully illustrates how the erosion of religious principles affects the entire fabric of society.
King Pariksit threatening to kill Kali for attacking the bull of dharma
Śrīla Prabhupāda comments:
When the king asked the bull who his assailant was, the bull did not blame anyone directly. Then the king confirmed his identity with these words: O you, who are in the form of a bull! You know the truth of religion, and you are speaking according to the principle that the destination intended for the perpetrator of irreligious acts is also intended for one who identifies the perpetrator. You are no other than the personality of religion. (SB 1.17.22)