QPR4 would be used when you still are doing the sampling. I.e. the technician doing the sampling damaged the labels somehow. Or they dropped a bottle and broke it and have to pull another sample. The key is primarily the sample drawing sheet/instructions and the custody of the samples. By using QPR4 you are indicating that all the samples drawn under that sample number where drawn at the same time, by the same person, and followed the same chain of custody. Same sample drawing number, just more samples.
In QPR6 you are doing a separate sample drawing, hence a new sample drawing number. You might do this for similar reasons, (broken bottle, maybe you reran tests several times and ran out of sample, maybe a sample got contaminated). In any case it requires some one to make a second trip. To draw more sample from the original quantity. These samples are taken at a different time, maybe by a different person. The chain of custody is different. But you still intend to test these samples within the same existing inspection lot that the earlier samples were tested with.
FF