ver. The interest you pay is always calculated by the principal times the interest rate. Paying extra money pays down that principal and thus your interest. You payment will stay the same however and the extra money not going to interest will go towards principal and shorten the amortization period of your loan.

I agree with you that I wouldn't pay extra principal on my mortgage if I had a 4% or lower interest rate. There are much better options to invest extra money in. I do only 30 year loans and will take as long as possible to pay those things off.