If you were to come up with a list of Samsung Galaxy S4 accessories, there are a lot of things you would likely put on that list - like cases, battery packs, screen protectors, headphones and speakers. That's a good start to a list, and many of those are things you will probably want. Maybe you even came up with a few other items not listed. Unless you're photography enthusiast, however, you may not have heard of lenses for your smartphone's camera. If you have a nice DSLR, you probably don't even use interchangeable lenses with your regular camera, if you use a standalone camera at all. Many people now just use their phones for photography.
The extensive use of smartphones for photography is why some companies have begun producing lenses for phone cameras. These lenses come in a variety of styles, and are made to attach to your phone directly over the camera. Usually this prohibits use of a case. Using a lens allows for optical zoom, something that the tiny cameras put in ultra-thin modern smartphones don't have room for. Though some cameras offer limited digital zoom, this reduces resolution and often lowers image quality. A camera lens will allow you to take advantage of the powerful camera in your phone, while still being much more portable than a bulky DSLR. It's probably not one of the Galaxy S4 accessories you've considered before, but it is one you'll enjoy, and one that will prove to be very useful for your phone.