From http://eventespresso.com/topic/paypal-item-id-ee4-paypal-standard/

add_paypal_item_id.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 'FHEE__EEG_Paypal_Standard__set_redirection_info__arguments', 'jf_ee_add_paypal_item_id' );
function jf_ee_add_paypal_item_id( $redirect_args ) {
    if ( ! defined( 'EE_MER_PATH' ) ) {
        $checkout = EE_Registry::instance()->SSN->checkout();
        if ( $checkout instanceof EE_Checkout ) {
            $transaction = $checkout->transaction;
            if ( $transaction instanceof EE_Transaction ) {
               foreach ( $transaction->registrations() as $registration ) {
                    if ( $registration instanceof EE_Registration ) {
                        $event = $registration->event();
                        if ( $event instanceof EE_Event ) {
                            $meta_key_value = get_post_meta( $event->ID(), 'paypal_item_id', true );
                            if ( ! empty( $meta_key_value ) ) {
                                $redirect_args[ 'item_number_1' ] = $meta_key_value;
                            }
                        }
                    }
                }
            }
        }
    }
    return $redirect_args; 
}