All 4
the top 2, never the last button
No. The tradition of leaving the bottom button of a coat undone started as a fashion choice. Coats are designed to flare away at the bottom, since the hips are (generally) broader than the waist,...
Either button two only, or all buttons.
Double Breasted - All Single Breasted: 2-button: Top button 3-button: Middle or first two