The Siemens 5SY8104-8 is a 1-pole SENTRON miniature circuit breaker with a D tripping characteristic, rated 4 A at 440 V AC. The 50 kA breaking capacity per IEC 60947-2 tells you it can interrupt a fault current up to that level without welding its contacts or cascading upstream; that's a serious SCCR for a 1-module-wide MCB.
Fit and integration
Snaps onto standard DIN rail, 18 mm per pole (one modular unit). Mount in any position; the quick-assembly system means you can gang it without tools.
If you're freezing a BOM, this part is a stable line item.