The amount of interest you pay every month should change as the amount you owe on the loan deccreases. Paying 4.125% on $347,900 is going to be a lot different than paying 4/125% on $200,000. 

The amount that goes to principle should increase with every payment and the amount that goes towards interest should decrease, see this amortization calculator here: http://bretwhissel.net/cgi-bin/amortize