The block observatory (block n°4) is equipped of a VDP "cloche" big model assuring the faraway observation
with its periscopes (two M, a N more a P2 and an emergency P8) and of a gap observatory for the direct surveillance of the Resistance Position. This gap was solely in fact an intended very heavy armoured to receive the system of hopper of GFM "cloche". The means of observation of this gap were therefore the episcope of infantry, and the block binocular D which makes an auxiliary observatory armoured. The plans of fires of defence expected the use of a FM over in this gap.
Unfortunately this material has been destroyed during the war and the repairs done by the military engineers in the immediate after war consisted merely in a replacement by a classic FM gap . The restoration of this system constitutes one of our objective to term.

The block 4 doesn't possess local underground nor proper battery of filters and its access gets used directly to the level of the main gallery by a very long passageway (more than 200 m) then a staircase in staples of about twenty meters clearing directly in the room of the observers, that is the lower floor of the block, and above to the superior floor from where one has access directly either to the gap or ito the VDP "cloche". This block is extremely tiny, each of the levels having a surface of about 2 m by 2.50 m. It is more an integrated observatory in the "ouvrage" than an actual block.

The FM allowed to beat the whole glaze of the tray, among the top of the South flanking block, of the block turret and the surroundings of the chimney.