Hi Eric,
This is where having a Moodle development test site comes in handy. On mine, the latest version of Questionnaire works in the new Moodle 4.0 with one bug that may or may not impact on it's use for you. It produces one debug message regarding completion:
*_get_completion_state() callback functions such as questionnaire_get_completion_state have been deprecated and should no longer be used. Please implement the custom completion class mod_questionnaire\completion\custom_completion which extends \core_completion\activity_custom_completion.
line 729 of /lib/completionlib.php: call to debugging()
line 666 of /lib/completionlib.php: call to completion_info->internal_get_state()
line 295 of /mod/questionnaire/questionnaire.class.php: call to completion_info->update_state()
line 75 of /mod/questionnaire/complete.php: call to questionnaire->view()
Otherwise everything seems to work okay.