In menu item I did add CSS class for page.
But this class is not added to BODY tag.
I checked index.php of Jasmine template and there I replaced this code
$pageclass = $menuParams->get('pageclass_sfx','');
by this one:
$pageclass = $menu !== null ? $menu->getParams()->get('pageclass_sfx', '') : '';
(I took above code from Cassiopeia template)
Now pageclass is added.
Maybe you want check your J4 templates regarding this issue?
Patrick