Cron: Update four post at Hour

This code will call your function every hour.

// The 'if' condition is needed to make sure the scheduler runs only once
if ( ! wp_next_scheduled( 'my_custom_action' ) ){
    wp_schedule_event( time(), 'hourly', 'my_custom_action' );
}

// Here we create our own action to attach to the scheduler
add_action( 'my_custom_action', 'update_content' );

It is also recommended you clear all scheduled events when you deactivate the plugin like so:

// The deactivation hook is executed when the plugin is deactivated
register_deactivation_hook( __FILE__, 'my_deactivation' );
function my_deactivation() {    
    wp_clear_scheduled_hook( 'my_custom_action' );
}

Note that if your website has zero visitors the cron will not run, image taken from here:

You can read more about wp_schedule_event() here.