Igor

extension String {
    func components(withLength length: Int) -> [String] {
        return stride(from:0, to: self.count, by: length).map {
            let start = self.index(self.startIndex, offsetBy: $0)
            let end = self.index(start, offsetBy: length, limitedBy: self.endIndex) ?? self.endIndex
            return String(self[start ..< end])
        }
    }
}

let str = "123456789"
let comp1 = str.components(withLength: 4)
let comp2 = str.components(withLength: 3)

dump(comp1)
dump(comp2)