The ABB S201-D4 is a 1-pole miniature circuit breaker (MCB) with a Trip Curve D characteristic, rated 4 A at 277 V AC. The D-curve means the magnetic trip is set to activate between 10x and 20x In (40–80 A instantaneously), designed for loads with high inrush current — motor starters, small transformers, solenoid valves, or lighting ballasts where a C-curve would nuisance-trip on startup.
The 1-pole width (roughly 17.5 mm) keeps panel density high — you can pack 16 of these in a 12-inch rail segment. Rated for 277 V AC phase-to-neutral on a 480Y/277 V wye system, so it's a direct fit for North American lighting panels where each breaker feeds a single-phase branch.