Is there a ubuntu server minimal installation? or is just 4gb/6gb base of installation without the packages, other feature installation?
There is the minimal CD: https://help.ubuntu.com/community/Installation/MinimalCD, only ~40 MB. This allows you to install a minimal system, and select different setups.
Ubuntu Minimal results in the same files being installed on the drive as the standard installation; it simply downloads the files rather than including them in the ISO.
You may want to consider using lubuntu instead. It uses the same core system as standard Ubuntu, but stripped down and with a more lightweight desktop environment.