I love it how everyone seems to be gladly jumping into the train on rails towards another evolution debate...

Do we know for sure that the four-legged chick was caused by a mutation in a control gene?
It could've been simple conjoined twins.
Essentially, evolution cosists of mutations. There are harmful, neutral and useful mutations. Harmful mutations don't generally get a chance to spread in population, while useful mutations give a better chance to stay alive and procreate. Neutral mutations just hang along, and they usually aren't noticed until they become either harmful or useful.
If the chick was a mutation, it was arguably a harmful one - hence it died away.
Dolphin having extra pair of fins could be either useful or neutral mutation. Most likely it was neutral for the dolphin - the fact that he or she was caught is more related to chance.
All mutations are part of evolution process. Evolution process doesn't actually touch individuals - individuals don't evolve, the species does.
By the way. Statistically, it is extremely unlikely that the only four-finned dolphin in the world was caught. There are others most likely like this one. And I don't think it's as much as a mutation bringing out old traits. It's more like the fact that dolphins might have lost their "legs" later than is generally thought. Perhaps (read: likely) there is a part in dolphin population that still has two extra fins.