Is it possible to restore Hyper-V virtual machine (just one of 4) using Windows Server Backup (if it was backed up first)? Or is there no way to do individual and every time one machines goes bad full recovery of Hyper-V is required? I've read http://support.microsoft.com/kb/958662/en-us but I do hope that at least some of those things are possible to workaround.
The backup solution is for backing up running VMs on-the-fly through the hypervisor/host, so it has its limitations.
If you need individual VM fidelity, then you can consider running the backups from within the VMs themselves. With this route there is a lot more freedom to chose the backup solution(s) that meet your specific requirements. The possible downside may be in having to support multiple backup technologies depending on your choice/needs.