Molex has +5V and +12V as well as ground, but the adapter to PCIe obviously just uses the +12V and ground leads.
You should have no problem adding the 4th GPU. However, don't expect a less-than-enthusiast power supply to last years and years with a heavy sustained load. Unfortunately the warranty is only 2 years.
Actually, you're drawing a little more DC power than you may expect, because Raidmax is apparently a bunch of lying ****heads with regards to this unit:
That's kinda disappointing given what this system is being used for, but it's alright. I never expected diamonds from a raidmax PSU.
It may not last much past 2 years, but that's alright, if it does it does if it doesn't it doesn't.