Does the +4 bonus during a backstab stack with the +2 against a surprised opponent?

I’m not an Autarch, but I am pretty sure it is not.

One of the most obvious ways to set up a backstab is by gaining surprise. In effect, the bonus gained from surprise is already subsumed in the bonus gained from backstab.

Yeah, this is what I figure, too. Usually, backstab situations give anyone a +2 to hit, so I figure the +4 is basically “+2 and another +2.”

That’s correct. It’s +2, or +4 if a thief/backstabber.