Howdy Tammy,
I would love to help you with this. That is not an issue I have seen. You can see here that version 4.0 does have the same responsive mobile calendar: http://wpshindig.com/events/
It is likely that the reason you were not seeing it on your site is because of a conflict between the latest version and something on your site, probably with your theme. Running 3.12 is a good idea while you wait for a fix. We are planning to support 3.12 for a while because we want to give theme and plugin authors time to update their plugins to support 4.0. But, they will need reports from users like you letting them know things are broken so they can begin work on fixes. Later on when you update to 4.0 again, if you continue seeing an issue try doing a before you downgrade to 3.12. This guide walks you through how to test for a conflict, and then identify what is conflicting. Once you know what is conflicting, let the author of the plugin or theme know so that they can fix it.
Does that all make sense?
Cheers!
– Brook