See LSJ s.v. tetraktu/s, I, for the Pythagorean sense of this headword. (Sense II, with various subsets, relates to musical intervals.) See also tau 481.
[1] The headword is identically glossed in the Synagoge (tau121), Photius' Lexicon (tau194 Theodoridis) and Lexica Segueriana 385.21; cf. Hesychius (tau625) and Photius, Bibliotheca 439a 5-8. The tetraktys represents the summation 1+2+3+4=10 geometrically -- typically with pebbles, dots, or alphas -- as one item centered above a pair, three underneath those, and a base of four, thereby forming an equilateral triangle (Ross, pp. 145-6; Annas, p. 54-5). The sums of successive levels comprise the triangular numbers: 1, 3, 6, 10, and so forth (Burnet, pp. 113-4; Thomas, pp. 86-91).
[2] The preceding three sentences also appear, approximately, at alpha 3880 (end). Number naming traditions varied. Parts of one can be gleaned from Aristotle's extant works and the commentary on the Metaphysics by Alexander of Aphrodisias, the latter evidently working from Aristotle's lost work on the Pythagoreans. For example, the number one is mind and being; two is opinion; three -- comprising beginning, middle, and end -- is the whole; and four represents both justice and, as the tetraktys itself, the whole nature of number (Burkert, p. 467; Ross, p. 144-5).
[3] Swearing their oaths upon it (Ross, p. 146), the Pythagoreans considered the tetraktys perfect; cf. epsiloniota 29 notes. Other Suda passages devoted to Pythagoras include pi 3120, pi 3121, pi 3123, and pi 3124.