"The things which are" pertains to seven letters John was to send to seven literal churches in Asia Minor (2:1 - 3:22). Each describes the spiritual condition of the church, and each contains the present tense phrase, "He that hath an ear, let him hear what the Spirit saith unto the churches." But, while the seven letters were to be written to seven literal churches, those same letters were addressed to all "churches." In other words, because all Scripture is profitable for all believers (2 Tim. 3:16), the letters, being Scripture, were to be sent to all churches during the Church Age, not just to the one to which each was addressed.
"The things which shall be hereafter," begins with John being told to "Come up here, and I will show thee things which must be hereafter" (4:1). Since this is the last time the Church is mentioned as being on earth (see 22:16), those believing in a Pre-Tribulation Rapture, see John as representative of the Church, and his being immediately in the Spirit in heaven (4:2), as a picture of the Rapture, thus ending the Church Age. Today, Christians are awaiting the Lord's return for us (Jn. 14:1-6; 1 Thes. 4:13-18; etc.), and our Rapture will trigger "the things which shall be hereafter."
While we do not know for certain how much time will pass between the Rapture and the beginning of the seven-year Tribulation (Dan. 9:27; Mt. 24:4-28; Rev. 6 - 18), it is clear that Revelation 4 - 5 describe the preparation for its beginning. Taking John's vision sequentially, during the first half (Rev. 6 - 11), there will be Seven Seal Judgment, and Seven Trumpet Judgments prior to the Satanic Trinity (Satan - Ch. 12; the Antichrist and the False Prophet - Ch. 13) initiating what Jesus called "The Great Tribulation" (the second half - Rev. 14 - 18).
To be continued.