Four empowerments (Tib. wang shyi; Wyl.dbang bzhi) — in general, and according to the inner tantras, there are four levels or stages within any empowerment:
the vase empowerment (Wyl. bum pa'i dbang; Skt. kalaśābhiṣeka),
the secret empowerment (Wyl. gsang ba'i dbang; Skt. guhyābhiṣeka),
the knowledge-wisdom empowerment (Wyl. shes rab ye shes kyi dbang; Skt. prajñājñānābhiṣeka) and
enables the student to attain the svabhavikakaya level of realization.
Further Reading
His Holiness Dilgo Khyentse Rinpoche, Pure Appearance—Development and Completion Stages in the Vajrayana Practice (Halifax: Vajravairochana Translation Committee, 2002), pages 1-14. restricted publication
Thinley Norbu, The Small Golden Key (Shambhala Publications, 1999), pages 60-63.
Tulku Thondup, Enlightened Journey (Boston: Shambhala, 1995), Chapter 12 'Receiving the Four Empowerments of Ngöndro Meditation'.