def osr_proj4(input_osr):
    """Return the PROJ4 code of an osr.SpatialReference

    Args:
        input_osr (:class:`osr.SpatialReference`): OSR Spatial reference
            of the input projection/GCS

    Returns:
        str: Proj4 string of the projection or GCS

    """
    return input_osr.ExportToProj4()