This is pretty much a no brainer...
1st, you have the GF selling out on store shelves without a major competitor, and the V5 is the main competitor to the GF, not the V4...
2nd, Since the V5 uses 2 chips, they can only produce half as many V5's, so to get the competing product out on the market in volume they devoted all the yields to the V5...
the V4 is a Low end product, these sell well, but they don't offer the most profit, the V5, even with 2 GPU's and 2 banks of RAM still offer more profit than the V4, because for the V4 to be viable it has to have a low price...
Craig