for much less than $4K ($2400 to be exact), i would build what i have
i3 3220T
4 X MSI Radeon HD7970 3GB OC
Corsair AX1200
i - the "i" is important, means you get 80 PLUS Platinum to lower electricity cost, without the "i", its Gold only
those cards made it trivial to get a %10 overclock on top of the factory overclock
i ended up shutting it down as it eventually dawned on me that i live in an old house with old electrical cabling and running a server, gaming P.C. AND a 1.1kW miner is likely cause me to become a victim of an electrical fire while i slept
while it was running (without any optimization i would have done if i continued to use it), I was getting nearly 2.7 GH/s
i don't like the odds but where are you on the planet? i've been unsuccessfully trying to find someone that can house my miner, share the risk by covering electricity and i give them half what it makes