Is it possible to install Intel 4th Gen CPU?

If the CPU you're talking about was never an Apple OEM part used on the 2012 boards, NO. You'll have firmware and driver issues. If. however. it was offered as an OEM "upgrade" should you be willing to do all the work (its a very advanced DIY project) it would probably fit.

FWIW you'd probably be better of economically to just sell you old machine and buy a new one with the processor you want. You have to locate and purchase the new CPU, try to sell the old CPU, later maintain and sell a custom box.

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