Maybe you just need to update the BIOS. Your i7-4790 is in the CPU support list
for your motherboard, but it requires BIOS
version P2.0 at a minimum.
That could be the problem!
Reboot your computer and look for information on the POST screen (the one with all the white text on a black background) about your current BIOS version. Tell us what it says. If you have an old BIOS version, then that's probably your problem.
Anyway, if you go to your motherboard's page, there's a tab for BIOS updates, leading to this page
You'll see the latest BIOS version is 2.60. Go with that.
You'll also see there are three different versions of version 2.60: "Instant Flash," "DOS," and "Windows." Those just refer to three different installation methods. You'll see an (i) symbol next to each one. Click it, and you'll see instructions for how to install that particular version. I think the Windows one looks like the easiest, and Instant Flash looks like the second easiest. The Windows version is an EXE you just execute within Windows, and it will update your BIOS. With the Instant Flash, you put the new BIOS on a USB flash drive, reboot the computer, press one of the F-keys, and update the BIOS from the flash drive. The DOS version looks like the hardest, because you'd have to install DOS to a flash drive, and I doubt you have DOS installation media sitting around.
So start with the Windows version and see if it successfully updates your BIOS. If it does, then you should be able to try the new CPU without a problem.