4x4 is an attempt by AMD to deal with Intel on the highend now that Conroe is out. Since their architecture scales much more easily to multiple sockets they're saying they're going to release consumer 2-socket systems that use regular (not registered DDR2), in motherboards that aren't crazy expensive. It's much cheaper for motherboard manufacturers to produce 2-socket K8 mobos than it is to produce those expensive Woodcrest Dempsey mobos. And let's not even talk about the price of FB-DIMMs. Apparently you should be able to get a pair of coherent hypertransport (SMP) enabled Althon FXs for <~US$1000. Oh ya, and the motherboards arre going to be enthusiast-oriented. i.e. you can actually overclock them.
The price of the CPUs is still steep though. If I could put a pair of X2 3800+s in there, it'd be great, but US$500 / chip is way beyond what I will spend on a CPU these days.