Exapostellō (ἐξαποστέλλω) means to send forth — a compound of ex (out from) + apo (away from) + stellō (to send/equip). It carries the sense of authoritative dispatch — sending on a mission. Galatians 4:4 uses it for God's sending of the Son; 4:6 for sending the Spirit.
Galatians 4:4-6 contains two of the most concentrated exapostellō usages: "God sent [exapostellō] his Son" and "God sent [exapostellō] the Spirit of his Son." The double sending — Son and Spirit — is the structure of salvation: the Son accomplishes redemption, the Spirit applies it. Both are divine missions, both originating "from" (ex) God. This passage is one of the NT's clearest Trinitarian structures. The verb insists on both distinction (the Sent is not the Sender) and unity (the Sender authorizes and identifies with the Sent).
The Trinitarian pattern of exapostellō in Galatians 4 mirrors John's sending language: "As the Father has sent [apostellō] me, I am sending [apostellō] you" (John 20:21). Mission originates in the inner life of the Trinity — God's "sentness" is prior to the church's mission. The church's apostolicity is derivative: we are sent by the one who was sent by the one who sends.