Solved the issue. here is what i did;
Go to BIOS and update the bios to 1.D version
then go to Settings>Advanced>PCI subsystem Setting
Above 4G Decoding set to Enabled


But very important:
I was using Kabylake CPU (Pentium G4560)
Because of that i failed. Then i tried Skylake series CPU i3 6500 and boom. i mined with 7 GPUs.

If you use Kabylake u can mine only with 4 GPUs
I was in trouble because of that.