Yeah, unfortunately, if you upgrade from 3.x to 4.x without saving backups of your files or, at the very least, your database then you're out of luck. You can't downgrade.
4.x has a learning curve to it but I assure you, it's the best series Invision has released to date and 4.3 / 4.4 are the crown jewels to this point. 4.5 has a lot of potential and 4.6 is right around the corner.