Are you sure the GPU hasn't already been sitting in the 16x slot and you making the mistake now of actually putting it in the 4x slot?
Get your mobo manual and check where the 16x lane is, in all manuals it is pointed with a black dot and a black line or just a black bot and a number to it, check to see which is the 16x slot.
If it is the 16x slot, dust it off, I suggest you use the vacuum cleaner with horse hair addon if it has it, then make sure to remove any possible static electricity from it before you turn it on, do this only if you don't have compressed air.
Alternatively get some plastic you can blow into, with a small, tight opening on the other end and blow the dust that way.
If you still have problems, remount the GPU again, make sure you close the "strap", otherwise the GPU may not even work in some cases and if it does work it can cause all sort of problems. Also make sure to screw any screws that there are to provide a stable positioning for the card.
If those don't work, you can flash your bios to the previous version and see if that helps, there are many tools out there that do it.
Alternatively you can check the card in someone else's computer and see if its card related.