Is this plugin ready for WooCommerce 4.0.0.?

Yes, it is.

When I try to upgrade WooCommerce, there is an advise that tell me that the compatibility of this plugin with WooCommerce is Unknown.

That’s new feature of WC and WP to give you a heads up. You may safely upgrade, don’t worry. In case you stuck somewhere, you may report me here.

Thanks for your help!