Swift 3: How to know if a set certain amount of strings match in 2 arrays

I have a simple line of code with 2 String Arrays. They both contain the same strings inside both arrays and I have an if statement that will work if both arrays are the same. Like so:

var firstArray: [String] = ["Music", "Art", "Sports", "Movies"]
var secondArray: [String] = ["Music", "Art", "Sports", "Movies"]

if firstArray == secondArray {

   //they match...
}

However, I want to be able to add a few more strings to the first array so the 2 arrays aren't fully the same, which will cancel out the if statement above BUT I want to run the if statement if 4 or more strings are the same in the second array as they are in the first array. How do I do this? Thank you.