But I think you might be interested in this
http://img29.imageshack.us/img29/2140/413307hisradeonhd6970ic.jpg
That's HIS's custom design, and I think this particular design has a great advantage for your situation, as it can draw air above AND below the fan. So the fan gets air from two directions thanks to that opening, this should be very useful when you sandwich 4 cards next to each other, you will need to give the fans as much spacing as possible for optimal cooling and this particular design helps exactly with that.
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/44826-his-hd-6970-iceq-turbo-hd-6950-iceq-x-turbo-x-review-15.html
http://www.tweaktown.com/reviews/4133/his_radeon_hd_6970_iceq_turbo_2gb_video_card_review/index15.html
Here's a couple of reviews that claims the cooler gives lower temperature than reference design. That IceQ cooler is probably your best bet.
(But as I said, it will get hot when you have 4 cards)
If you're talking about the 6950, then you are only doing half of the explanation, some of them went with custom designs to improve the cooling, while some of them went with custom designs to lower the production costs.