the problem with quad sli is not the volume itself, but rather the cooling and power.
assuming you have a strong enough power supply, and that the GPU will be OCed to best be used for mining,
you need to think about a water cooling solution. otherwise, you will not use the GPUs to full extent.
and if that is the case, it wont be cheap, but a decent full tower will do the trick.
you can start here:
http://www.hitechlegion.com/reviews/cases/10569-thermaltake-level10gt-case-reviewnaorai