The Servant in Isaiah 50:4 speaks of the Lord GOD.
This is the only one of the Servant passages where the term occurs, and it occurs four times in the song (see further Isaiah 50:5–6, Isaiah 50:9). It emphasizes the gravity of the Servant’s mission and the certainty of his vindication, for it is none less than the almighty Lord himself who has called.1 It is also noteworthy that the term does not appear in either Isaiah 50:1–3 or Isaiah 50:10–11, which lends further support to the notion that one ought not to include either of those in a unit with Isaiah 50:4–9.
4 The Lord GOD has given me the tongue of those who are taught, that I may know how to sustain with a word him who is weary. Morning by morning he awakens; he awakens my ear to hear as those who are taught.