This is what i got from them about 1 month ago regarding the same issue...
Greetings *******
Thank you for contacting us.
There is no way to add additional slots onto an existing account. 4 is the maximum per one account. However it is possible to purchase and own more than one account, which would each have 4 character slots per account.
To purchase Guild Wars online, please check our site at
http://www.GuildWars.com to download and
http://www.PlayNC.com to order a registration key.
You would need a second different email address for the other account. If you do not have one, there are several sites that offer free email addresses such as Yahoo or Hotmail.
If you have any further questions, please do not hesitate to ask us.
Take care, and happy hunting!
Regards,
Guild Wars Support Team
Guild Wars Home Page:
http://www.guildwars.com
Self-Help Knowledge Tool:
http://support.guildwars.com
Terms of Use:
http://www.guildwars.com/termsofuse.html