I enjoy playing characters with high comeback potential like Makoto, Seth, Gouken and even Oni and El Fuerte. Except for Jago, who is easiest to learn for SFIV players? Which character in KI has the most similar to SFIV metagame? (Like how Jago somewhat plays like Ryu.) Thanks!

If you take a look at my character page, you can turn on an "Ease of Use" rating in the first graph, or look at how the characters compare in the "Ease of Use" stat in the bar chart. These are just my opinions of course, but give you a pretty good idea.
Some good suggestions for comeback characters are:
Jago: Probably has the most SF4-esque gameplan in the game. Once you become comfortable with the combo engine and learn a few KI-specific tricks (neutral jump can bait throws, for instance), you'll feel right at home. Instinct mode is a game changer for comebacks, because he will regenerate tons of health and he can hit confirm into it easily off all his good tools.
Fulgore: Most like Seth in the game. Great zoning, some of the trickiest mixups, can play very similar to a strong SF4 character if you learn a few small set plays after enders (meaty fireball, teleport stuff). If Poongko played KI, he'd probably pick Fulgore.
Spinal: Another really strong momentum character that can steal games with overwhelming offense. I'd argue he plays the least like a SF4 character out of all the ones I listed here, though... he has to do some run cancel stuff and his neutral game takes a bit of getting used to.
Thunder: The best raw damage in the game and the best command grab in the game. He struggles to get in sometimes, but he can apply very basic SF4-esque mixups and they all hurt.