This verse tells us that these two are the two olive trees and the two lampstands "standing before the God of the earth." Why are they described as "the two lampstands"? Timing is vital to understanding this. Revelation 10 and 11 are internally chronological. At this time, the seven thunders have ceased, and the Two Witnesses have been raised up. They are the sole effort God has going as far as witnessing, preaching, and proclaiming His way on the earth.
What does a lamp do? It gives light (Matthew 5:14-16). What are the Two Witnesses doing at this time? Revelation 11:4 says that they are the two lampstands that stand before the God of the whole earth. What are they doing? They are lighting the whole house, as it says in Matthew 5. What is the house? Who comprises the house of God? The church! The two olive trees put their oil in the reservoir, and it feeds the whole church - to do what? To make light! At this time, though, the church is hidden in a Place of Safety, and not even Satan can get to them, as far as we know. We know that certainly no men can get to them.
So, we could say that the church's light is at that time under a basket. Who is left to be light to the world? The Two Witnesses! They are, at this point, the two lampstands. All the eyes of the world will be drawn toward these two prophets. They are the only ones that will be doing good works at that time; they are the only ones that will be publicly glorifying God in heaven.
That is why they are called the two lampstands. They are the only ones remaining to shine spiritually during Jacob's trouble and the Day of the Lord. They will be, in effect, raising Cain all over the world. The whole world hates them, and they will rejoice when these two are dead - because they cannot stand the fact that these two shine so brightly for God.
At this point, the seven churches are out of the picture, so the lampstands cannot represent churches. They picture these two bright lights for God. Not only will they be supplying the church with oil, but they will also be shining brightly as witnesses to the world as a result of the good works that they do.