The Siemens 5SL3204-7MB is a SENTRON miniature circuit breaker (MCB) — a 2-pole, C-curve device rated 4 A with a 4.5 kA breaking capacity at 400 V AC per EN 60898.
The 4 A rating (at 30 °C, per) governs the continuous load current — wire this into a circuit drawing over 3.2 A continuous and the thermal trip will eventually open. The C-curve means magnetic trip occurs at 5–10× In (20–40 A), so it passes brief motor-start surges but clears a hard short fast. Breaking capacity of 4.5 kA at 400 V AC (per EN 60898) means it can safely interrupt a fault current up to 4.5 kA without welding contacts or rupturing the case. For installations where the prospective short-circuit current exceeds that, step up to the 5SL4 series (10 kA) or add a current-limiting upstream device. Two poles, two modular-width units (36 mm wide, per and). That's the DIN-rail footprint: it occupies two adjacent 18 mm slots in a distribution board. Depth is 76 mm with 70 mm installation depth — verify gland-plate clearance if the enclosure is shallow.
Rated for overvoltage category III (, degree of pollution 2), which covers fixed-installation distribution boards downstream of the main service entrance.