import requests

def fetch_pr_diff(owner, repo, pr_number, token=None):
    headers = {
        "Accept": "application/vnd.github.v3.diff"
    }
    if token:
        headers["Authorization"] = f"token {token}"

    url = f"https://api.github.com/repos/{owner}/{repo}/pulls/{pr_number}"
    response = requests.get(url, headers=headers)

    if response.status_code == 200:
        return response.text
    elif response.status_code == 404:
        return "Error: PR not found. Check the repo and PR number."
    elif response.status_code == 401:
        return "Error: Unauthorized. Check your GitHub token."
    else:
        return f"Error: Could not fetch PR. Status code: {response.status_code}"