A half-life of two hours means that at the 2-hr mark, there would be only half the original amount in the bloodstream. At the 4-hr mark there would be half the amount present as at the 2-hr mark--so 1/4 of the original amount. After another 2 hours it would be down to half again, so only 1/8 of the original amount. So depending on the original levels in the blood, after 4 hours there could well be detectable amounts of excess CPK in the blood. 
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"We are--each of us--dying; it's how we live in the meantime that makes the difference."
"It's not what you gather, but what you scatter that tells what kind of life you have lived!"
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