Your computer's maximum bandwidth should at least be 100Mb/s. So even 12Mb/s doesn't even come close to maxing out your PC bandwidth. To be honest, I guess it has to a lot to do with the difference in cost as to whether it is worth it to you. The speed of sites based on CMS systems like Drupal are more dependent on the host's performance than the client bandwidth. If you plan on sharing the connection with multiple computers or plan on doing a lot of torrent downloads, I would definitely go with the 12Mb/s connection.
4Mb/s is going to be fine for what you mentioned. If the cost difference is significant, you could go with the 4Mb/s and always upgrade to the 12.