The VS 4 is a very old design, its basically a balanced piston driven first stage and came with a choice of second stages, (ie) basic or with an adjustable option.
It was a so, so breather and suffered from a few inherrant issues like "whisteling" on inhalation as air flowed over the HP seat, and as Robdiver says they didnt tune very well due to the fact they never really got the HP seat material and size quite correct so IP pressure was a bit of a hit and miss affair.
The second stage was also rather an old design with a small adjuster which could be added as an optional extra, the adjuster is simply a spring and rod which presses on the LP lever screw putting back pressure on the LP seat, they did this because the 2nd stage in its basic form was in fact the same as the VS3 which suffered from free flows, this lever increased pressure on the seat and stopped the free flow - a bit cheap and crude, but it worked I guess.
That said, its a basic workhorse, a bit crude around the edges manufacture wise and an oldish design, but will probably be fine for normal recreational open water diving. Not sure if its still supported spares wise though.
If you need some schematics for parts or maintainance etc, PM me and I will e-mail them to you.
