Even fixed rate loans are amortized - which means while you're payment doesn't change, the amount applied to interest vs principle changes with every payment. In the beginning, much more is going toward interest than principle. There are amortization calculators that will give you an estimate of how your monthly payment is applied every payment or year, that should give you a better illustration.
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
Honeslty you shouldn't get a refund, you should adjust your withholdings so that at the end of the year, you owe $0 and you get back $0. Wouldn't you rather have $300 per month in your pocket and use it as you need throughout the year vs letting the government borrow $3600 from you interest free? It is nice to get a big chunk like that back at once too though, because if you keep it monthly, it'll probably go to misc. things and you won't feel it as much.
Yup I love to get a very little refund at the end of the year. Means I maximized my income all year.