In the formal epistolary salutation starting in v. 4, John specifies his audience as the seven churches in the province of Asia. In doing so, he was not suddenly shifting his perspective from the broad to the specific (i.e from everywhere to the province of Asia), but was introducing symbolism to refer to the same audience he mentioned in v.1. The seven churches in Asia represent the entire church of Jesus Christ.
When a numeric like seven is repeated descriptively, as it is in v.4 , there is no way to miss its obvious symbolic implications. Throughout the scriptures, not just in the Apocalypse, this number is used to indicate completeness, or entirety (e.g. this). Seven days completed creation, seven has stood for completion in regard to God's work ever since. The only question to answer in regard to the symbolism of v.4, is to what entirety or completion does it refer?
I would say the force of both references (v. 1 and 4) is meant to convey that the address is to all the church throughout all time. That discounts the preterists viewpoint, for the scope was not limited to the Roman persecution of the ancient church, but addressed to the church throughout the ages covered by the prophecy. When the final judgment and the resurrection are mentioned as a piece in the continuum, how can can such a narrow time frame be justified?
Furthermore, it discounts interpreting the letters to follow as successive ages of the Church, for the churches are addressed corporately as a whole in chapter one. True, they will be dealt with in isolation from each other in the body of the letters to follow, but contemporaneously--distinct but at the same point in time. Whereas the seals, trumpets and vials have a time progression built into them (one is broken, blown, poured after another) the letters do not. They were sent out at once, together, to be read by all simultaneously.
The seven churches are both real and representative. Real in that there were seven such churches in John's day that faced issues such as written about in the letters. Representative as demonstrated by the symbology: not of periods of time, but of differences in the character of typical local churches which make up the one church universal. The seven churches, then, is merely a synecdoche which refers to the entire church through all time.