"""Parse PostgreSQL URI and extract connection details.

Parses a PostgreSQL URI string and extracts the connection details such as host, port, user, and database. 
If any of these details are not provided in the URI, default values are used:
- host defaults to 'localhost'
- port defaults to '5432'
- user defaults to 'mirix'
- database defaults to 'mirix'

Args:
    uri (str): The PostgreSQL URI string to be parsed.

Returns:
    Dict[str, str]: A dictionary containing the extracted connection details with keys 'host', 'port', 'user', and 'database'.
"""
You should write code starting with:
from urllib.parse import urlparse
from typing import Dict

def parse_pg_uri(uri: str) -> Dict[str, str]:
    # TODO: Implement this function
    pass

