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.
"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!"
"Be kinder than necessary, for everyone you meet is fighting some kind of battle."