The Siemens 5SY6304-8 is a SENTRON 5SY6 miniature circuit breaker, 3 poles, D-curve, rated 4 A at 400 V AC. This is the high inrush version — D characteristic means it holds through motor start and transformer energisation without nuisance tripping, so it's the right call for inductive loads in mechanical engineering and industry. For UL 1077 / CSA C22.2 No.235 installations it's rated 5 kA. Occupies 3 width units (54 mm).
Mounting position any, so orientation in the panel doesn't restrict airflow or cable routing. Combined terminal top and bottom — line and load can enter from either end, which speeds up wiring in a crowded enclosure. Sealable cover option allows the breaker to be locked out after commissioning.