Smartism is the odd one out among the four, and calling it a denomination is the least accurate of the four uses of that word.
The other three are defined by whom they worship. Smartas are defined by what they accept as authoritative: the smriti, the remembered literature (the epics, the Puranas, the dharma texts), held alongside the Vedas, and read within the framework of Advaita Vedanta, which teaches that the individual self and the ultimate reality are finally not two. Smartism is therefore closer to a Brahminical orthodoxy, a way of holding the whole inheritance together, than to a devotional movement with its own god.
Its characteristic practice is the panchayatana puja, the worship of five deities together (Vishnu, Shiva, the Goddess, Ganesha and Surya), arranged around a chosen one. The point is doctrinal rather than diplomatic. If the ultimate reality is nirguna, without attributes, then the gods are forms it takes for the sake of the worshipper, and choosing one implies nothing whatever about the others. A Smarta can worship Shiva on one day and Vishnu on the next without inconsistency, because on this account neither is finally what is being worshipped.
The tradition attributes both the philosophy and the practice to Adi Shankara. The attribution of the philosophy is fair. The attribution of the five-deity worship is not: arrangements of five shrines are attested archaeologically from the Kushan period, well before Shankara on any dating of him, so the practice was received rather than invented.