The first three chapters is a rather idealistic picture of the church--one that can help us see what we should be working towards.
The last three chapters are in Martin's terms "ecclesiology brought to earth"; that is, some harsh realities--harsh, yet not insurmountable.
My heavily reworded summary of his summary of 4:1-16:
1. Be true to your destiny while remembering that unity is essential.
2. Unity does not mean that we are all alike.
3. Church members have different gifts.
4. Christ intends for grownups to be grownups.
5. Christ intends for the church to be grownup.