If they are above the water line and easily replacable it doesn't matter. Go for the cheapest. A4 is definately the best but it's also the most expensive, after all what price difference are we talking about here? /forums/images/graemlins/smile.gif
Its better to use A4 everywhere but it must be marine grade I would guess that that means passivated. If its not passivated then they both corrode, the term is called an excess of chlorides.
A4 should only be 20% dearer than A2. But mention marine and the price will double.
According to Ian Nicolson's Boat Data book 304 passivated is next best to 316 passive. 304 active is just a little better than copper on the voltage scale.
Should you ask the chandler,yes, will he know, I doubt it. A specialist shop might, or ask Plastimo or Ronstan or Barton what type of steel is their kit made from.
For deck use, except where seawater might get trapped. I believe A2 is fractionally stronger than A4, as well as a fair bit cheaper. If you are passionate about shiny, then A4 will stay bright longer, A2 needs occasional cleaning to hold slight discolouration at bay.
A4 is 316, which has 2% molybdenum over A2=304, to resist pitting by chlorides. If the component is likely to suffer a strength reduction by pitting ( and possibly therefore initiation of fatigue) then use A4. This would cover items such as rigging. If the component is unlikely to suffer significant strength reduction or is replaced/disassembled frequently, then use A2.
All stainless steel is passivated, this only refers to its tendency to form an oxidised film in air. The only time stainless steel can be active is when it is being corroded, particularly in crevice corrosion. The values given for active/passive in the galvanic series are largely theoretical for normal use.