If reading A4 PDFs is high priority, you'd probably be best with a tablet PC or a notebook at present, to be honest.
The price of eInk machine is controlled by the cost of screen manufacture. At present, a 6" screen runs at about $200 to manufacture; an 8" screen 2-3x that. That would probably put the price of an A4 screen at around $1500-2000, at which price point the market would, I suspect, be very limited.
Screen prices will inevitably fall as production ramps up. In a couple of years I'm sure we will see A4 devices at sensible prices, but not yet.