def bbox_union(bbox1, bbox2):
    """
    Extract the union of two bounding boxes.
    Input:
        bbox1: 4-tuple of (xmin, xmax, ymin, ymax) for the first region
        bbox2: 4-tuple of (xmin, xmax, ymin, ymax) for the second region
    Output:
        Returns a 4-tuple of (xmin, xmax, ymin, ymax)
    """
    output = []
    for i, comparison in enumerate(zip(bbox1, bbox2)):
        if i % 2 == 0:
            output.append(min(comparison))
        elif i % 2 == 1:
            output.append(max(comparison))
    return output
