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Can your Motherboard fit the XP-120? THAT is a heatsink. I'd recommend that one, but if you can't do that then I'd go with copper. And yes, 4 GHz on air is impossible. That would be tough on water-cooling as well though it could be done. The only way you'd be guaranteed to hit the 4 GHz mark is with Peltier or Phase Change cooling.
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