The altar that is front of GOd's throne.
Context, context, context. The setting for the opening of the seals is established 2 chapter earlier:
After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter.And immediately I was in the spirit: and, behold, a throne was set in heaven, and one sat on the throne. - Revelation 4
This is where John is when the seals are opened, before God's throne. So when the souls of the dead martys are "beneath the alter" we know they are in the or under the throne room of The Lord.
Just go head 2 more chapters and we see:
And I saw the seven angels which stood before God; and to them were given seven trumpets. 3 And another angel came and stood at the altar, having a golden censer; and there was given unto him much incense, that he should offer it with the prayers of all saints upon the golden altar which was before the throne.
I don't know why you persist in ripping scriptures out of context to keep making these points that are not supported by Scripture. The altar is not symbolic. It is actually and literally before the throne of God. The martyrs are not symbolic. They are real born again believers who were killed for their faith. And yet here are their souls, speaking, full aware and conscious. Not asleep.
This one passage alone ends the discussion. The scripture could not be plainer. Will you finally yield and just admit that you are not rightly handling the text? Where is the humility? Where is the fear of The Lord over just winning an argument?