This structure was completely below ground with raised entrances that would have been level with the surface at the time of construction. Subsequent development raised ground-level, totally obscuring any trace of the structure. As a result of this, shelter F4 was also damaged by contractors during their sub-surface excavations and it was not possible to obtain complete measurements for the two entrances. Once shelter F4 was located and fully exposed, it was observed that the shelter was roughly aligned north to south. The northern entrance was 65cm above the roof of the chamber and 1m wide, with a surviving length of 5.5m descending at least eight steps into a corridor. This was capped by concrete-covered corrugated iron that sat on a lip in the entrance walls that was 5cm deep at both the sides and chamber end. The southern entrance was destroyed completely up to a point just outside of the
chamber and was 1m wide, with a surviving length of 4.7m. There was evidence of at least four remaining steps which had descended into a corridor. Fragments of concrete with corrugated iron adhering to them would suggest that this entrance was also capped. Both corridors were 70cm wide and terminated in recesses that were 87cm deep and contained a centrally-located exhaust vent (10cm in diameter). The floors in both corridors were of poured concrete with five 60cm by 46cm slabs set into them. A white painted number (48) was observed on a facing panel of each entrance.
The central chamber was identical to shelters F1-F3 in terms of measurements and fittings. <1>