A solution is `substringFromIndex`

 let a = "StackOverFlow"
 let last4 = a.substringFromIndex(a.endIndex.advancedBy(-4))

or `suffix` on `characters`

 let last4 = String(a.characters.suffix(4))

code is Swift 2

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**Swift 3:**

In Swift 3 the syntax for the first solution has been changed to

 let last4 = a.substring(from:a.index(a.endIndex, offsetBy: -4))

---

**Swift 4:**

In Swift 4 it becomes more convenient:

 let last4 = a.suffix(4)

The type of the result is a new type `Substring` which behaves as a `String` in many cases. However if the substring is supposed to leave the scope where it's created in Apple recommends to create a regular `String` instance.

 let last4 = String(a.suffix(4))