i want to hook my iphone 4 to my powerbook what osx do i need

You don't. Snow Leopard 10.6 requires an Intel processor.

You need a Mac with an Intel processor.

Usually it is a specific version of iTunes that is required. In all likelihood this will be iTunes 10.6 which requires OSX 10.5 or higher. A Powerbook can run 10.5 but maybe not happily (mine takes a minute or two to boot). We can provide more suggestions if you provide more specifics as to exact phone model requirements (it says so on the box), and exact computer specifications.

To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

If you do decide to upgrade to Leopard, be prepared to spend a lot for it. It costs as much (or more) used than it did new, and you will not find Apple selling it.

I'm just going based upon the OP's statement:

it says my osx has to be upgraded to 10.6

You could very well be right that he really just needs 10.5.

More bells and whistles may require OS 10.6, or even 10.7 for full iCloud interaction. Basic syncing requires whatever iTunes version is specified on the box.

Leopard 10.5 Technical Requirements - http://support.apple.com/kb/HT3759

List of Applications Not Compatible with Leopard - http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard

Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/3636662

"a brody" post about upgrading to Leopard - https://discussions.apple.com/thread/3373795

You have exactly the model and specifications PB I have that runs Leopard but takes two minutes to boot. Depending upon how much you are going to have to spend on Leopard, and any possible harware upgrades that may be necessary you may think about shopping for a newer model computer.