Arc 1 is Trails in the Sky through Trails in the Sky 3rd, then Arc 2 is Trails from Zero and Trails to Azure, and Arc 3 is Trails of Cold Steel 1 through 4. Trails of Reverie finishes off the story for Arcs 2 and 3. Arc 4 is Daybreak, which just came out in English

Alternatively some people start with Cold Steel 1+2 before going back to the previous arcs, but personally I'd recommend against that, because Arc 1 takes place a few years before 2+3, and 2+3 take place side-by-side, so Arc 3 references Arc 2, and they both reference Arc 1