Of course you can use all 4 mem slots, if you couldn't they would not be their, right?
X79 is a chipset (southbridge) not a CPU. If you could wait for Haswell, that is what I would suggest, the Z87 is still a 2 channel chipset like the Z77. The X79 supports CPU's with 4 mem controllers (quad), it has nothing to do with Haswell, it is the CPU itself. Ivy Bridge CPU's come in both flavors, some duel channel and some quad channel, just like Haswell will. But quad is way expensive and not needed for a game rig.
Sure, wait a couple weeks and get the new stuff, I think you would be better off in the long run in doing so. That is what I am doing now, waiting for a i5-4670 and a Z87 MB, with a new GTX780 or 770 it should be good to for...