recheck all your power wiring...also when you try and start it what happens, does it beep, do you have flashing lights on the MB...there should be a indicator to you that you can cross check against your MB manual to see what is wrong (make sure you have the 4 pin inserted correctly (I think I have one that is an 8 pin that breaks into a 4 pin and I noticed I did not have it set properly which of course meant no boot up)..
I had a similar issue with a build I did back in December. I built a rig with an A4-3300 APU and 8GB of Corsair Vengeance (Low Profile) 1333mhz ram. The rig would work fine with a dedicated graphics card, just didn't work with the onboard graphics.
Try taking out one stick of ram and see if your system posts. And if you can exchange your ram, go and get it exchanged for an 8GB kit that's listed under the supported ram list It'll save you a lot of headache.
Thanks for the help. Finally got it working the other night. The 4-pin connector worked just fine. Turns out I was either tightening the board too tight or it was somehow contacting the case and shorting.
After I pulled it out of the case, if fired right up. I just put some rubber washers between the mobo mounting screws and the mounting studs. Works like a charm!