SB 4.23.32
brahmano brahma-varcasvi
rajanyo jagati-patih
vaisyah pathan vit-patih syac
chudrah sattamatam iyat
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
If one hears of the characteristics of Prthu Maharaja and is a brahmana, he becomes perfectly qualified with brahminical powers; if he is a ksatriya, he becomes a king of the world; if he is a vaisya, he becomes a master of other vaisyas and many animals; and if he is a sudra, he becomes the topmost devotee.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
In Srimad-Bhagavatam it is recommended that one should become a devotee regardless of one’s condition. Whether one is without desire (akama) or with desire (sakama), or whether one desires liberation (moksa-kama), he is advised to worship the Supreme Lord and execute devotional service unto Him. By so doing, one attains all perfection in any field of life. The process of devotional service — especially hearing and chanting — is so powerful that it can bring a person to the perfectional stage. In this verse brahmanas, ksatriyas, vaisyas and sudras are mentioned, but here it should be understood that that reference is to the brahmana who is born in a brahminical family, the ksatriya who is born in a ksatriya family, the vaisya who is born in a vaisya family, and the sudra who is born in a sudra family. But whether one is a brahmana, ksatriya, vaisya or sudra, he can attain perfection simply by hearing and chanting.
To take birth in a family of brahmanas is not the ultimate finishing touch; one must have the power of a brahmana, which is called brahma-tejas. Similarly, taking birth in a royal family is not the all in all; one must possess the power to rule the world. Similarly, taking birth as a vaisya is not all; one must possess hundreds or thousands of animals (specifically cows) and rule over other vaisyas as Nanda Maharaja did in Vrndavana. Nanda Maharaja was a vaisya who possessed nine hundred thousand cows and ruled over many cowherd men and boys. A person who is born in a sudra family can become greater than a brahmana simply by accepting devotional service and giving aural reception to the pastimes of the Lord and His devotees.