Did you miss the 'interleaved' word somehow? You need all 4 hashes to have last 4 bits zero to match the target 0x0000FFFF.. I'd suggest just reading the code.
However, using only last 64 bits of each of 4 hashes (and effectively only last 8-10 bits for PoW at current difficulty) kills the math behind their cryptographic security proofs.
That's does not need to be bad to just take a portion of a hash. But if one of the algorithms for Heavycoin is broken, you simply do not need to apply it any longer. You satisfy a higher difficulty with less effort. If the same algorithm for Quark is broken, you do not win anything. And that's why Quark is more secure. That more than obvious. Heavycoins claims the opposite. Claiming 1+1 = 3 is pretty much at the same level.