For the media set that both tapes belong to, did you set the overwrite protection period to a a period of time? Otherwise, the tapes can become overwriteable immediately after they are used.
Also, check your Media Management Options (go to Tools ----> Options -----> Media Management). If it is set to the second option,
Overwrite recyclable media in the media set before overwriting scratch media, then what you are experiencing is as expected.
If the above two conditions are present, your "Week4a" is used and it becomes part of a media. However, the media set overwrite protection period is set to 0 hours, then it becomes overwriteable immediately. If your Media Management Option is set to Overwrite recyclable media, then it will be asked for first before "Week4b" tape which is still a scratch tape.
You might want to read the following documents
http://seer.entsupport.symantec.com/docs/299050.htm
http://seer.entsupport.symantec.com/docs/192265.htm
which will explain the various media set settings better.
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If this clears up the mystery, please mark it as the solution.