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First off: I don't think linking to sites where you can get programs and such that you would normally have to pay for, for free, are acceptable.
Second: I've been using CS4 for a while, and aside from a better looking interface, I haven't seen much.
There's a content aware scaling feature that's nice, and another that you can merge photos together (like if you had a camera on a tripod and took pictures as you rotated it to make a huge panorama picture.
That's all I can think of off the top of my head.. If you don't use any of those features, or ANY of the ones CS4 brings, it's not needed to upgrade.
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