John Calvin considered the possibility that the fourth man was Christ, but settled on identifying the man as an angel instead.
Charles Spurgeon preached that the fourth man was the pre-incarnate Christ:
V. The last and perhaps the most pleasing part of the text is, WHO WAS WITH THEM IN THE FURNACE.
There was a fourth, and he was so bright and glorious, that even the
heathen eyes of Nebuchadnezzar could discern a supernatural lustre
about him. “The fourth,” he said, “is like the Son of God.” What
appearance Christ had put on I cannot tell, which was recognizable by
that heathen monarch; but I suppose that he appeared in a degree of
that glory in which he showed himself to his servant John in the
Apocalypse, and such was the excessive splendour and brightness, the
God-like air that was about him, the flash of his eye and the
splendour of his gait as he walked the fire with the other three, that
even Nebuchadnezzar could not help saying he was like the Son of God.
Barnes argues that the original Chaldee did not include the definite article "the", suggesting "a Son of God" not "The Son of God". But he also comments on the popularity among scholars of the view that this was a Christophany:
Was it an angel, or was it the second person of the Trinity, "the" Son
of God? That this was the Son of God - the second person of the
Trinity, who afterward became incarnate, has been quite a common
opinion of expositors. So it was held by Tertullian, by Augustine, and
by Hilary, among the fathers; and so it has been held by Gill,
Clarius, and others, among the moderns.
I imagine that many other theologians have come to opposite opinions on this.