I would indeed recommend to update the plugin to the latest version first. Is there a specific reason you don’t want to update MC4WP?
Other than that, turn on “log everything” under MC4WP > Other, test the integration and let me know what shows up in the log.
Also, check if it’s set to double opt-in under MC4WP > Integrations > WooCommerce checkout, if it is, of course the subscriber needs to click the opt-in link before showing up in Mailchimp.
Hope that helps. If you have any questions, please let me know!