Troubleshooting overheating issues with my cracking rig

Hello everyone,

I've recently invested in a dedicated password cracking machine, but I'm having trouble getting the most out of it because of heating issues. Here's a picture of how it looks inside.
Basically, that's 32 GBs of RAM, a Core i7 and 4 AMD GPUs (SAPPHIRE R9 290 4G GDD5 TRI-X).




Now as soon as I tried cracking some passwords with it, the overheating problems showed up. No matter what I tried, it would never run for more than 5 minutes. I figured that the GPUs were too close to each other and were blowing out their heat on each other.
What I did next (while being quite unhappy about it) was remove two of the four GPUs; this way, there's some space between them for the air to circulate. I still had major overheating issues, so my last ditch effort was to place the machine in front of an opened window and add a big general purpose fan in front of it:



Now, it works a bit better. I still get interruptions due to overheating every 1-6 hours, but at least I can get some work done.
It feels quite bad to have 2 GPUs that I cannot use, though, so I was wondering if anyone would be so kind as to give me some advice on this problem. Thanks a lot in advance for your time!