Bob Utley offers these remarks:
"the fourth is like a son of the gods" A considerable amount of speculation has been made about this fourth personage. He is an angel of the Lord (cf. Dan. 3:28 also 6:22). It is humorous to note that when Nebuchadnezzar calls the boys out by name, he makes no mention of that fourth person!
The Anchor Bible Commentary, Daniel 1-9 (by L. F. Hartman and A.A. Di Lella) says for Daniel 3:25: "a divine being. Literally, ―a son of God,‖ rightly understood in vs. 28 as an angel; cf. also vs. 26."