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My Powerbook is more than 2.5 years old. I cherish it so much. I hope it keeps lasting. The past 1.5 years it is only a secondary computer I use approx once per week, so hopefully that helps prolong its life.

I love it since the form factor is no longer made. It's actually the lightest mac notebook ever still. ha-ha.

I say wanting to use a current Mac Book 4 years is not bad idea, you never know what might happen, but hopefully be ok. After all, already about 6 months old. 1/8 of your goal is complete. :)
 
My iBook is over 3 years now and have been in use almost everyday. I mostly use it for surfing, mail, iWorks, Rapidweaver and organizing photos.

I have to upgrade to a MBP soon because I need the Exprss card slot for my new camera, I´ll keep the iBook around for a long time still I think.
The only thing I changed was the battery
 
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