CPU for ipad and iPhone 4

Looks like all the devices are running the A4. Read more about it on here

"The chip commercially debuted with the release of Apple's iPad tablet;[4] followed shortly by the iPhone 4 smartphone,[5] the 4th generation iPod Touch and the 2nd generation Apple TV. It was superseded in the second-generation iPad, released the following year, by the Apple A5 processor."

As for replacing it, you would basically have to do the same as the reballing for other IC's so you better have the right equipment. Check the links on this answer for an idea of what you would be getting into. The other problem you may run into is where to get the A4 and the price you have to pay for it....all a matter of economics.