What Will Happen If I Installed A Graphic Card In A Pciex16 Slot, Running At X4 ?

I am planning to buy Gigabyte motherboard GA-970-Gaming and it has one PCI Express x16 slot, running at x16 and the other one PCI Express x16 slot, running at x4.
And I have got two graphic cards:
1. Radeon HD 6870
2. nVidia GT 520
I want to install the Radeon HD 6870 on the PCI Express x16 slot, running at x16 and the GT 520 on the PCI Express x16 slot, running at x4, so will the performance of the GT 520 drop, and could it cause damage to it over time.

 

PCI Express x4 should be enough for GT520. No damage should come. Just wondering how you like to use those cards simultaneously. You cannot combine them into one faster card but with that configuration you can get more display connectors.

 

Thanks very much.
Yes, I want to connect another screen.
So, does the screen connected to the GT520 will take it's graphical power from the GT520 ?

 

Display takes graphic processing from GT520. There might be some problems with dual video cards from different manufacturer though. If not, then that way you can have more displays.