from typing import List
from typing import Optional


def get_default_projection(supported_projections: List[int], selected_projections: List[int]) -> Optional[int]:
    """
    Gets a default projection of either 4326 or the first supported projection.
    """
    if 4326 in supported_projections and 4326 in selected_projections:
        return 4326
    for supported_projection in supported_projections:
        if supported_projection in selected_projections:
            return supported_projection
    return None