Clear Code Encryption Implementation

# Introduction

In this challenge, we will be implementing a clear code encryption algorithm. This algorithm is based on the historical practice of encrypting telegraph codebooks, commonly used in intelligence agencies during the Qing Dynasty in China. The objective is to take a four-digit number as input, add a predefined encryption key to it, and return the resulting four-digit encrypted code. If the input number is not a four-digit number, or falls outside the range of 0 to 9999, the function should return `None`.