I nearly went Office Space on my mining rig.
For a brief period of time I had three GPUs mining at 37.1 MH/s, so I tried to hook up the fourth and go for ~50 MH/s. Not only did it not work, but 2 or 3 no longer worked and I was stuck back on one GPU running at 12.4 MH/s. I spent hours tinkering until finally I was able to get two GPUs mining again. More tinkering and I was able to get three GPUs recognized and running, but only two GPUs mining.
Three GPUs detected during startup
Three GPUs beginning to run
You can see the fans spinning on all three GPUs
Yet the 3rd GPU would drop off (while still running) and 24.8 MH/s gets recorded
Another hour spent unplugging everything, plugging it all back in, switching wires around, and I got three to work: currently running at 37.1 MH/s. The fix ended up being switching the 2nd GPU with the 3rd GPU -- I have no clue why that fixes it, but welcome to the mining game.
I want to try to get 4 running, but given what I've went through to finally get three to work I'm not sure I want to deal with it. That said, my profitability with three running is likely about $0.008 per hour, so it's not like the downside is that bad. But my goal is to basically get .2 ETH mined -- to hit the minimum payout for my pool -- and to either quit, or mine different coins, so at least this way I would mine it somewhat faster. Will report back in a few days when I'm overcome by boredom and boldness and go for the 4 GPU glory again.
My name is Ryan Daut and I'd love to have you as a follower. Click here to go to my page, then click 
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I am a professional gambler, and my interests include poker, fantasy sports, football, basketball, MMA, health and fitness, rock climbing, mathematics, astrophysics, cryptocurrency, and computer gaming.