The heatsink is called the Coolermaster GeminII and not only are they annoying, they are very prone to breaking the traces within a motherboard. Because of how much they weigh, even with the rear bracket, it was just too heavy!
The bottom 4 screws you took a picture of should be the ones to remove it. its that or you'll need to remove the motherboard to get it out! I would get that cooler out anyway though, get something like the Arctic Freezer, its only like 29 bucks and is very good for the price!
Anymore questions, just ask!