I have a brand new S4 with all factory settings. Up until yesterday I installed several Apps. Today when I try to install an App from the Google Android App site, it says it's installed but it's nowhere to be found on my phone.
Look in the apps folder. You may need to reorganize it by selecting the menu button and choosing something like alpha. Apps folder is on the bottom right of the screen. It might be named only part of the full name of the app si look for parts of the title.