How can I access lock screen Apps on Android like iOS 7 -- Galaxy S4 on Sprint
For security reasons, you can't access normal apps from the lock screen: you can only access lock screen widgets. They're like home screen widgets, but the app they come from has designated them as suitable for the lock screen: they may have better ways to keep sensitive data hidden than normal home screen widgets.

Not all Samsung phones support lock screen widgets. For example, the Galaxy S III doesn't (unless you install a 3rd-party lock screen app, or a custom ROM), because its lock screen was designed before Google added lock screen widgets to Android. The Galaxy S4 does, though, so you just need to add the widget of your choice to it!