Counterstruck with the number "4" for what ever reason. Not an error. Welcome to Coin Talk! I posted a couple of oldies here: http://www.cointalk.com/t149836/
Not on a common modern coin. But there are collectors for the older ones especially if the marker can be identified (generally on coins from the 18th and 19th century). Some counterstruck coins have unusual marks. For example, some of the Spanish coins have another denomination struck onto a larger denomination coin. Those can be quite collectible.