As a “long term support” (LTS) release Hardy Heron is supported for three years on the desktop and for five years on servers.
The forthcoming update is relatively modest with only 9 bugs targeted. These range from Nautilus not displaying Samba shares for certain networks to specific hardware fixes for Dell machines, which, of course, ship with Ubuntu 8.04 on certain configurations.
A full list of targeted bugs can be found @ launchpad.net/ubuntu/+milestone/ubuntu-8.04.4