Q: Powerbook G4

Sorry, but you can't.  See this tip:

https://discussions.apple.com/docs/DOC-6021

I should amend that, for those things you need iTunes 10.6.3, you'll need to upgrade your Powerbook to 10.5.8:

http://support.apple.com/kb/dl1575

10.5.8 is not free though, and it disables Classic, which may or may not be a concern for your Powerbook as this tip mentions:

https://discussions.apple.com/docs/DOC-1757

10.5.8 is available through these steps:

https://discussions.apple.com/docs/DOC-2275

1. You need to upgrade the computer to Mac OS X 10.5.8; read #2 before upgrading the computer. If desired, the retail DVDs are available from sources such as Amazon or eBay.

2. It isn't possible to upgrade a PowerPC Mac such as that to a Mac OS X or iTunes version new enough to sync an iPhone 5C, so you'll need a newer computer.

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No. Apple never released that version on a retail DVD.

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Yes.

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Apple menu -> System Preferences -> Accounts lets you establish multiple accounts to load iTunes from, each with their own library.

Yes, or you can launch iTunes with the Option key held down and create a new library. That pane was renamed in Mac OS X 10.7.

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