I also have a Asus A8N-SLI Deluxe, and yes, it sees all 4GB. You suffer a bit of a performance hit by installing four 1GB memory chips because it forces the memory controller to figure out which of the memory banks it has to go to for each read and write. So there is no point in buying the fastest memory you can find.
By using a 128MB graphics card I am able to utilize about 3.6GB of my RAM. The rest of the 4GB address space is used by device-associated memory (like the graphics card).