FES-4 is the “cross reed” low-profile reed switch from Fujitsu. FES-4 is likely to have been introduced to meet German DIN ergonomic standards, before being abandoned in favour of the cheaper leaf spring family. The Fujitsu timeline suggests that FES-4 was introduced in 1980, which seems a little early for a DIN-compliant product. Curiously, the series name was assigned out of sequence, using the previously unassigned FES-4 designation.
While the older reed types all mount the reed capsule vertically (as was traditional), FES-4 places it horizontally, as found in European reed switches from FR Electronics, ИЗОТ and Unitra Dolam. Like ИЗОТ reed switches, the large reed capsule is placed diagonally across the centre of the switch and extending from corner to corner, making it single-pole only (FR Electronics and Unitra Dolam reed switches use a smaller capsule that is offset to one side, permitting double-pole arrangements).
As with all other Fujitsu reed types, FES-4 is rare.