I see that some site forms use the ZIP+4 method of an input, while others just use ZIP. How often do people actually know their ZIP+4? Also, what companies really benefit from this or require it? Just debating the best route for a typical ZIP input.
The best route is to allow both. If someone knows it and wants to enter it, it doesn't hurt you at all.
Zip+4 makes it easier to have things sorted when shipped out. For some companies that are shipping lots of items, this can save on shipping costs if they agree to presort the things they are shipping. This typically only really happens with things like magazines or other bulk mailers (ads, catalogs, bills, etc.).
I did a quick search on how to find your Zip+4 code and there seems to be no definite way to find it (and the USPS page for it is broken). Most people seem to revert to looking at their mail to determine it (As per Yahoo answers). However with regards to if the Zip+4 code is really needed, I think this excerpt from Wikipedia explains it best :
Quoting from this discussion on a forum (Note: the responses are not supported by solid facts)
So considering that most of the web based inputs would printed if used for shipping or delivery notifications, you would not need a Zip+4 code as a normal zip code would suffice