A bond issued with a 7% coupon in a world that now wants 4% is worth more than face, because the above-market income stream has value. The price rises until the yield falls back to 4%.
The catch is that the premium disappears. Every coupon you collect is partly a return of the premium you paid, and at maturity you only get par-value back. Your current-yield looks generous while your yield-to-maturity tells the truth.
Example: a 7% bond with five years left, priced to yield 4%, trades near 113.4. Current yield is 7 / 113.4 = 6.17%, which flatters. Yield to maturity is 4%, because you will lose $134 per $1,000 of price over the five years.
Related: discount-bond, par-value, current-yield, yield-to-maturity