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The extreme 4 can do PCI-e 8x/8x in crossfire, while the extreme 3 can only do 16x/4x.
The extreme 4 has also a front usb 3.0 header, the extreme 3 does not. Oh, and you also noticed the all black color shceme for the extreme 3, while the extreme 4 has blue and white
Personally, after using virtually every 970 board available, I am partial to the Biostar TA970XE, and the gigabyte GA-970A-UD3. I would pick the TA970Xe for that build, as it is the fastest performing 970 board