Thanks guys for all the replies. After many hours and frustration the problem is solved, not by me but a mechanic shop.

I decided to take it in once the flex pipe replacement didn't help, but thank you for your help.
Turns out that the intake gasket on cyl # 4 was missing. I guess the Hyundai dealer that did all the work on the car before ****ed up and didn't put it on about a year ago. When there were some lifter problems.