if you have an ad format that displays 4 ads instead of 2, or something along those longs, would your EPC drop? as it is, the one that bids highest gets the spots, right? so if you have four ads instead of two, wouldn't the top four bidders show, and not the top 2?
It looks to me that it is in a rotation and I would assume the ones that paid more for their ads would display more (a benefit to both Google and the website which adsense is displayed).
Display more ads will give more option to your visitors on which ads they should clicks. So the probabilty they will click is higher when you display 4 ads rather than 2 ads
But keep in mind it also can make your site look cluttered. I suggest using the larger choice only if you have more than enough space.
I purposely create a space for advertising on my site. Its money dude, you should find a way to make it come into your pocket