What is the difference in meaning between the following sentences?
1. I am in church.
2. I am in a church.
3. I am at church.
4. I am at a church.

I am aware that 'in a church' means being inside the building called church. And being 'at church' can mean being near a church or inside a church. And being 'at church' means being in attendance in your local church. What is the difference between 'in church' and 'at church'?