exclude one child by id from $post->post_parent

Maybe it’s just a matter of parentheses.

if ( ( $post->post_parent == '4426' || in_array( '4426', $post->ancestors) ) && !is_page(4430)  ) { // Who We Are
    //do something
}

I’m assuming is_page accepts an Integer being the page ID and 4430 is not the slug or page title.