Third party Spotlight search was introduced in iOS 9 but unfortunately it is not supported on older hardware.

From Apple’s documentation:

App Search comes with iOS9, available as a free software update for iPhone 4s and later, iPod touch (5th generation), iPad 2 and later, iPad mini and later. However, the search functionality of NSUserActivity and CoreSpotlight are not supported on the iPhone 4s, iPad 2, iPad (3rd generation), iPad mini, and iPod touch (5th generation).