Thank you for the help,
 but I figured today myself that the issue was due to faulty risers also in my case...

I bought 4 powered 1x to 16x cables and two of them don't work properly... Plugging 2 gpus directly on the 16x slots and the remaining two with the working risers did the trick and now windows 7 detects all gpus correctly.

Seems a lot of risers cables sold nowadays are of poor quality and many people reports problems.


If can I say something about your rig's instability, could be due to lack of power, maybe your psu is not enough for 4 gpus? You should have at least a 1200w gold rated psu, with a 1250-1300w gold would be safer.

Btw thank you again for your response