The key risks are: 1. Prevailing interest rates can go way above 4%, leaving you with a less-valuable investment. This is what happened for most of the period.

The key risks are: 1. Prevailing interest rates can go way above 4%, leaving you with a less-valuable investment. This is what happened for most of the period. 2. Prevailing interest rates can go way below 4%, so your investment ought to be more valuable, but the issuer retained the right to call in and repay the bond at face value. This is what just happened.