I have a similar setup as you:

Hardware:
Asus P8P67 WS Revolution
Intel Pentium G620
Corsair AX1200 PSU
4x XFX 6870 (2x ZDFC and 2x ZDBC) -- no crossfire bridge
4z DVI homemade dummy plugs

Software/OS:
GUIMiner-20110614
AMD CCC 11.6 Drivers
Windows 7 Pro 32-Bit
GPU-Z v0.5.3

Are you using the latest AMD drivers, GUIMiner, and GPU-Z?

In the beginning I had problems where all 4x 6870's weren't able to run at 100%. When I ran it 4 at the same time, the hash rate would fluctuated between 50-150Mhash/s on all fours. Normally a 6870 OC'ed, I would get somewhere around 270-275Mhash/s (960/1175/1175v). Did some careful research and found out the problem and it was due to temperature being too high (90 degrees) which lowered the performance. Since you have those extenders, it shouldn't actually be a problem for you (maybe). I had to buy an extra fan and had it actually sit on top of the graphics card to lower it down to somewhere around 70-80 degrees. Temps for the graphics card shouldn't going higher than 85-87 degrees, IMO. If so, it's not good for the card.

As for the dummy plugs, I'm actually using this:

http://www.overclock.net/overclock-net-folding-home-team/366646-how-use-multiple-gpu2-clients-using.html

You can buy the DVI-to-VGA adapters for less than $2 from Monoprice.com. Resistors got them from Fry's for 99cents for a pack of 6 resistors.

Hopefully, this helps solve your problem