Combine WooCommerce order meta data in one meta data field.

functions.php

<?php

add_action(
	'woocommerce_after_order_object_save',
	function( $order ) {
		$data = [
			$order->get_meta( 'meta 1' ),
			$order->get_meta( 'meta 2' ),
			$order->get_meta( 'meta 3' ),
			$order->get_meta( 'meta 4' ),
		];

		$all = implode( ', ', $data );

		$order->update_meta_data( 'meta all', $all );
	}
);