As you can see in the Rig Veda Anukramani I provided in this answer, verses 4-6 of Rig Veda Book 1 Hymn 155 are addressed to Vishnu. Here they are:
We laud this manly power of him the Mighty One, preserver, inoffensive, bounteous and benign; His who strode, widely pacing, with three steppings forth over the realms of earth for freedom and for life.
A mortal man, when he beholds two steps of him who looks upon the light, is restless with amaze. But his third step doth no one venture to approach, no, nor the feathered birds of air who fly with wings.
He, like a rounded wheel, hath in swift motion set his ninety racing steeds together with the four. Developed, vast in form, with those who sing forth praise, a youth, no more a child, he cometh to our call.
The three steps seem like a clear reference to Vishnu's incarnation as Vamana the dwarf, but my question is, what is this reference to Vishnu's "ninety racing steeds together with the four"? What is the story of these 90 horses of Vishnu, and what does "the four" denote? Four additional horses?
Now Western Indologists, for instance in this book, have speculated that the 90 horses really denote the fact that there are 90 days in a season, and that the four denotes the fact that there four seasons in year. But are there any scriptures that describe Vishnu literally driving a chariot pulled by 90 horses?