What is the model number of your NGK spark plugs. I would put money on it that they are the NGK Platinum plugs but then again you have a four cylinder and that may be different.

As far as my theory about the price of NGK plugs.... The are the cheap looking plugs and not manufactured well at all. The outer electrode on the one I removed from my santa fe was way off center and it's clearly a manufacturer quality control problem. When I said NGK's are expensive I meant to regular consumers that purchase them at their local parts stores. Not that asian car manufacturers don't get them cheap for that is a no brainer and you know that they do.

Do a test. Look at a NGK Platinum and Bosch Platinum spark plug side by side. Feel them, smell them, taste them.....if you have any sense at all you will notice that the Bosch is a much better made spark plug!
GO CHIEFS!