<?php

function backorder_text( $availability, $product ) {

	$product_ids = array( 1, 2, 3, 4, 5 );


	if ( in_array( $product->get_id(), $product_ids ) ){

		foreach( $availability as $i ) {
			$availability = str_replace( 'Available on backorder', 'This item may take 3-4 weeks to deliver', $availability );
		}
	}


	return $availability;

}

add_filter('woocommerce_get_availability', 'backorder_text', 10, 2 );