Bug? sites on the same server sharing cache (worked on 3.0.1)

Hi,
Since I updated my object-cache.php to 4.0 (previously had 3.0.1) I have a mixed cache through 2 websites hosted on the same server (one site gets the db options from the other website). This didn’t happen when I had object-cache.php 3.0.1. If one of the websites have 4.0 and the other 3.0.1 everything is working again.

Thanks for your support.