Shadrach, Meshach and Abednego - who is understood as the fourth person in the fire? Shadrach, Meshach and Abednego, who is understood as the fourth person in the fire?

In the third chapter of book of Daniel after King Nebicanesor throws Shadrach, Meshach and Abednego into the furnace he sees four people in the furnace instead of three, who are talking with each other.


What is understand of who this person is? An angel, a Christophany?
What is the current Christian understanding of this? 
How would a Jewish person in ancient times have understood this?
Is there any extra biblical Jewish sources/tradition naming the fourth person (such as a specific angel, etc)?
The King said that the fourth person looked like "a son of the Gods."  This is translated as "the son of God" in the King James Version, which has led some people to conflate the fourth figure with Christ, but it's important to remember that the concept of "the son of God (singular)" makes no sense to the King, who was a polytheist.

He calls out to Shadrach, Meshach and Abednego and calls them servants of "the Most High God," which is to be understood literally.  He saw that their God was higher (more powerful) than the Gods he worshipped, none of whom had ever been known to protect someone from incineration in a furnace.

It's most likely that the fourth figure was an angel, much like the one sent to Daniel to pacify the lions just a few chapters away from this story.