There are multiple voice-recognition venues and options on the S4, and some of them definitely work better than others. For using voice commands to search, call, text, etc., Google Voice Search (AKA Google Now) works far, far better for me than Samsung's S Voice.
What I think most people in this thread are talking about is typing text, into a memo or text message for instance, by speaking into the phone. This will be dependent on the keyboard app you are using. Go into one of the apps where you commonly enter text by typing or by voice. Notice that in the status bar at the top left of the screen there is a keyboard icon, which indicates that you're in a text-entry app or area. Pull down the notification screen and find the item "Select input method." Tap on that to choose among, for instance, Google voice typing, Samsung keyboard, and Swype. (You may see different options, depending on the phone and on what you have enabled in Settings > My device > Language and input; here too the sequence or labels may vary on your phone.) Selecting Google voice typing gives you a voice-only text-entry system powered by Google's voice recognition. Selecting Samsung keyboard will bring up a keyboard that has a microphone icon just left of the spacebar. Tapping that icon will get you a different voice-recognition system to enter text. Other keyboard apps available in the Play store (such as
Smart Keyboard Pro, which I love) may also have voice-recognition activated by tapping a microphone icon. Some of these may just use Google, but others may not. The different voice-recognition systems definitely work differently for different people, depending on your voice, accent, etc.
So there are some choices. You can experiment (time-consuming, but worth it if you want to voice-type on your phone) and hopefully find one that works for you. But all of them are going to work best in a quiet environment when you speak clearly into the phone.