Should I upgrade to an iPhone 4 or wait for a newer version to come out?

I'm finally eligible to upgrade from my 3GS. There's nothing wrong with my iPhone and it works fine, but I'm getting that itch to upgrade. I love the retina display of the 4 and I'm torn between upgrading now or waiting it out, especially with all the rumors of a newer iPhone coming out this year. I'm not sure if there's any validity to these rumors swirling around. So should I wait or go ahead and just get the iPhone 4?