Using ‘posts_where’ on a query with a custom field

you don’t need to use posts_where for this kind of query , you can just use “meta_query“

$today = date("Ymd");   
$args = array(
    'meta_query' => array(
        array(
            'key' => 'TheDate',
            'value' => $today,
            'compare' => '>='
        )
    ),
    'cat' => '4',
    'posts_per_page' => -1,
    'orderby'  => 'meta_value_num',
    'meta_key' => 'TheDate',
    'order' => 'ASC'
);

$the_query = new WP_Query( $args );