SB 2.6.25

vastūny oṣadhayaḥ snehā
 rasa-loha-mṛdo jalam
ṛco yajūṁṣi sāmāni
 cātur-hotraṁ ca sattama
Word for word: 
vastūni — utensils; oṣadhayaḥ — grains; snehāḥ — clarified butter; rasa-loha-mṛdaḥ — honey, gold and earth; jalam — water; ṛcaḥ — the Ṛg Veda; yajūṁṣi — the Yajur Veda; sāmāni — the Sāma Veda; cātuḥ-hotram — four persons conducting the performance; ca — all these; sattama — O most pious one.
Translation: 
Other requirements are utensils, grains, clarified butter, honey, gold, earth, water, the Ṛg Veda, Yajur Veda and Sāma Veda and four priests to perform the sacrifice.
Purport: 

To perform a sacrifice successfully, at least four expert priests are needed: one who can offer (hotā), one who can chant (udgātā), one who can kindle the sacrificial fire without the aid of separate fire (adhvaryu), and one who can supervise (brahmā). Such sacrifices were conducted from the birth of Brahmā, the first living creature, and were carried on till the reign of Mahārāja Yudhiṣṭhira. But such expert brāhmaṇa priests are very rare in this age of corruption and quarrel, and therefore in the present age only the yajña of chanting the holy name of the Lord is recommended. The scriptures enjoin:

harer nāma harer nāma
 harer nāmaiva kevalam
kalau nāsty eva nāsty eva
 nāsty eva gatir anyathā