Adds the event location to the EE4 calendar tooltip. Requires EE4 and setting Display Attendee Limits to Yes in the calendar's Advanced Settings.

add_location_to_calendar_tooltip.php

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

function jf_ee_add_location_to_calendar_qtip( $text, $datetime ) {
    if ( $datetime instanceof EE_Datetime ) {
        $event = $datetime->get_first_related( 'Event' );
        if ( $event instanceof EE_Event ) {
            $venue = $event->get_first_related( 'Venue' );
            if ( $venue instanceof EE_Venue ) {
                $venue_name = $venue->name();
                $text .= '<br>Location:&nbsp;' . $venue_name;
            }
        }
    }
    return $text;
}
add_filter( 'FHEE__EE_Calendar__tooltip_datetime_available_spaces', 'jf_ee_add_location_to_calendar_qtip', 10, 2 );