Reconcile God's sovereignty with Satan being the god of this world In Matthew 4:8-9 we read about the temptation of Christ by Satan:


  Again, the devil took Him up on an exceedingly high mountain, and
  showed Him all the kingdoms of the world and their glory. And he said
  to Him, “All these things I will give You if You will fall down and
  worship me.”


In Christ's response, he did not challenge Satan's authority to make such a claim. 

Taken together with 2 Cor 4:4 where Satan is described as "the god of this world" it seems we can conclude that Satan is the ruler of the world.

How can we reconcile this with God's sovereignty?
This is a great question and one I had to take a few minutes to think through. I am Catholic and I think I have an answer for you- though there may be several others, possibly better ones.

Remember that Satan was one of the most powerful angels when he rebelled, and that he did not get stripped of his nature when he Fell. That means that Satan is an incredibly powerful being who, as seen in the Bible, can exert his angelic power in the planes of existence that humans inhabit (that is to say, the world of spirit and the world of matter). This is probably the way that Satan planned to follow through on his offer of world domination to Jesus.

Also, notice Satan wants Jesus to worship him. This implies that this is something he desires not only from Jesus, but from all humanity. What Satan has been doing since his Fall is pretend to be God Almighty, and treat the world like his playground. If Satan demands worship, this probably isn't the first time he tried to get it. This would be in keeping with St. Paul's comment "the god of this world". Who is the god of this world? Whatever everybody is worshipping. Be that other fallen angels, man's own desires, etc; it all leads back to Satan and his own agenda.

When Jesus comes to Earth, He's coming here like a rightful Prince come to reclaim His Father's throne from the usurper, Satan. God is sovereign, but that doesn't mean others aren't pretending that isn't so.