Add venue information to the Event Espresso 4 calendar tooltip. You can add this code to a functions plugin or, if available, into your WordPress child theme's functions.php file.

venue_calendar.php

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

add_filter( 
    'FHEE__EE_Calendar__get_calendar_events__tooltip_reg_btn_html', 
    'my_custom_calendar_tooltip_add_venue', 
    10, 
    3 
);
function my_custom_calendar_tooltip_add_venue( 
    $original_output, 
    $event,
    $datetime
) {
    $venue_html = '';
    if($event instanceof EE_Event) {
        $venues = $event->venues();
        if(is_array($venues)) {
            $venue = reset($venues);
            if( $venue instanceof EE_Venue ){
                $venue_html = '<div class="venue-tooltip">';
                $venue_html .= '' !== $venue->name() ? '<span>' . $venue->name() . '</span><br>' : '';
                $venue_html .= '' !== $venue->address() ? '<span>' . $venue->address() . '</span><br>' : '';
                $venue_html .= '' !== $venue->city() ? '<span>' . $venue->city() . '</span>' : '';
                $venue_html .= '' !== $venue->state() ? '<span>,&nbsp;' . $venue->state() . '</span>' : '';
                $venue_html .= '</div>';
                return $venue_html . $original_output;
            }
        }
    }
    return $original_output;
}