Providing that you'll use apache you'd better go with 40 IPs. I'm just quoting:
Name-Based VirtualHosts and SSL
As a rule, it is impossible to host more than one SSL virtual host on the same IP address and port. This is because Apache needs to know the name of the host in order to choose the correct certificate to setup the encryption layer. But the name of the host being requested is contained only in the HTTP request headers, which are part of the encrypted content. It is therefore not available until after the encryption is already negotiated. This means that the correct certificate cannot be selected, and clients will receive certificate mismatch warnings and be vulnerable to man-in-the-middle attacks.
In reality, Apache will allow you to configure name-based SSL virtual hosts, but it will always use the configuration from the first-listed virtual host (on the selected IP address and port) to setup the encryption layer. In certain specific circumstances, it is acceptable to use a single SSL configuration for several virtual hosts. In particular, this will work if the SSL certificate applies to all the virtual hosts. For example, this will work if: [...]