Trouble backing up iphone4 with older Mac Book

I need to sync and reset my iPhone 4 bc I'm getting a new phone for work and I need to hand this one back in. I have a PowerBookG4 and I've synced it dozens of times to my iPhone 4. But now it won't work. When I plug it in, iTunes says I need v 11.3 or greater. So I visit apple.com/itunes/downloads to download the 11.3.1 version, but nothing installs. A pop up window displays a message that states my architecture doesn't support the version of itunes. How do I resolve this? Buying a new Mac Book is not a solution, sorry Apple. I just need to get my data off my iPhone 4 so when I get my new iPhone 5 I can have all my contacts. I don't care about the pictures or apps, just need the contacts because I will be getting a new number. Thanks!