“So if these investors (going forward) feel they may be able to get a higher (and safer) competitive rate of 3-4%”

Just remember in a rising rate environment bond prices also carry risk. If you buy a bond paying 3-4% and rates continue to rise, the bond you purchased will also drop in value, unless you hold until maturity. 

Just like if you continue to hold ENB, you will continue to receive the dividend.  Except with ENB the dividend will rise. 

When was the last time a bond INCREASED its coupon rate before it matured?