substring of the first 4 characters from a textField in Swift 4

I'm trying to create a substring of the first 4 characters entered in a textField in Swift 4 on my iOS app.

Since the change to Swift 4 I'm struggling with basic String parsing.

So based on Apple documentation I'm assuming I need to use the substring.index function and I understand the second parameter (offsetBy) is the number of characters to create a substring with. I'm just unsure how I tell Swift to start at the beginning of the string.

This is the code so far:

  let postcode = textFieldPostcode.text

  let newPostcode = postcode?.index(STARTATTHEBEGININGOFTHESTRING, offsetBy: 4)

I hope my explanation makes sense, happy to answer any questions on this.

Thanks,