When is a Utility Knife to be preferred over a Chef's or a Paring knife?
There's no question that a 4 or 5 inch utility knife is going to see a lot less use than your chef's knife or your paring knife, both of which have innumerable uses.  The utility knife is a lot more specific, really being for cases where the paring knife is too short and the chef's knife is too heavy or thick.  I have a 4" utility knife, which (of course) I got for free with a 6" Sabatier chef's knife.  It gets used a couple times a week, for:


scoring onions in order to mince them
cutting limes in half (a chef's knife seems like overkill here)
splitting small peppers
slicing large California shallots
cutting cheese


... and similar occasional, accessory uses.  

Part of the problem with the 4" utility knife is that it's really a dwarf 6"-8" utility knife.  The 6" or 8" utility knife, now commonly called a "vegetable knife", is far more useful; the long, thin blade is excellent for making paper-thin vegetable and fruit slices, and can be used as a fileting knife if you don't have one.  At the 4" size, though, the utility knife is useful only for cutting things which happen to be small, and only if you don't already have a more general knife dirty.

If what you're really asking is "can I get rid of this knife?", the answer is "yes".