P55a-UD3 PCI-E Slot question

Thinking of crossfiring 2 x 6850's, since I've learnt that a x4 2.0 PCI-E SLI/Xfire is of hardly any noticeable disadvantage.

I just want to clarify something since the manual isn't clear.

On my P55a-UD3 there are 2 PCI-E slots.

x16 (2.0)

and a

x4 (???)

Can someone tell me whether or not the x4 slot conforms to 2.0 standard?

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If it doesn't I will probably reconsider xfiring.