The passage seems to contrast a man in the secular order with a man inside the Christian community:
KJV 1Jn 4:3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world. 1Jn 4:4 Ye are of God, little children, and have overcome them: because greater is he that is in you, than he that is in the world.
We know that Christ "dwells in our hearts by our faith" but how did the man [of sin] live in the people of the world?
Or, is it collective? Christ is where ever 2 or more are gathered in his name. Is John saying, "greater is he that is among you than he that is among the lost folks?"