did you buy the bond at par? if so there should be no gain or loss. if you bought at a discount the discounted amount you paid might have been less because going interest rates at that time was higher than 4%
say the going interest rate was 5% and you bought a 1 year bond you might have paid 9905
at maturity you get 10400 of which 400 is the 4% interest the other 95 is gain due discount
$9905 at 5% is $495 which you got $400 interest $95 excess of par over cost
