Please solve the programming task below using a self-contained code snippet in a markdown code block.

We know that 4 and 7 are lucky digits. Also, a number is called lucky if it contains only lucky digits.
You are given an integer k, return the kth lucky number represented as a string.
 
Please complete the following python code precisely:
```python
class Solution:
    def kthLuckyNumber(self, k: int) -> str:
```