There are several high quality exchanges coming online in the next month or so, including Coinsetter, TradeHill, & BTC Global. These will make a significant difference to the market.
Yes, Bitcoin needs more exchanges if we want to see higher prices. 4 big exchanges is not enough, we need 10.
Explain why we need more leading exchanges? Quantity over quality?
Stability... 10 big exchanges is better than 4 because customers funds will be more evenly distributed and diversified among the major exchanges. If one of the big exchanges goes down right now and loses customers funds, it would have a more severe impact than if it were only 1 of 10 big exchanges going down.
Hopefully some of these exchanges will help promote BTC by advertising? It can't hurt to have more options though and hopefully these new exchanges are more professional and make people feel more secure to buy BTC then the others.