You pay for 1 or 2 years of upgrades and support. So any updates in this time are free including Ability Mail Server 4 with the new database storage / WebMail etc. if it is released while your license still allows updates.
All Ability Mail Server 4 settings will be stored in database but because mails can be huge, mails will still be stored in file and referenced from the database but the status of mails and everything else will be database driven which should allow vastly improved performance and clustering scenarios.
We are also offering existing Ability Mail Server customers a 20% discount off Ability Mail Server 2011 until the end of August so if you email me your Ability Mail Server 2.xx order number, email address used to order or your license key to
chris@code-crafters.com then I'll send you the details of how to get this discount once your information is verified.