τετρακτύς was the Pythagorean name for the sum of the first four numbers (1+2+3+4=10), held by them to be the root of all creation.  cf. the Pythagorean oath: Ναὶ μὰ τὸν ἁμετέρᾳ ψύχᾳ παραδόντα τετρακτύν, Παγὰν ἀενάου φύσεως ῥιζώματ᾽ ἔχουσαν cf. my note on Theodoret, Ep. cxxx. for the use of τετρακτύς for the Four Gospels.