In a system plugin I have method public function onContentPrepare($context, &$article, $params, $page = 0).

In a com_contact.contact page view (frontend) the received $params are empty (null) which throws a fatal error when I use $params->get().

Plugin runs on J3 without issues.

I think $this->params in this line is wrong: https://github.com/joomla/joomla-cms/blob/4.0-dev/components/com_contact/src/View/Contact/HtmlView.php#L349

and should be $item->params: like this:

$app->triggerEvent('onContentPrepare', array ('com_contact.contact', &$item, &$item->params, $offset));

$item->params is filled step by step before the line, not $this->params

Compare also with Joomla 3:
https://github.com/joomla/joomla-cms/blob/staging/components/com_contact/views/contact/view.html.php#L306