When creating a CT there is a checkbox named "Activate quota" in the Root disc tab.
Further you can go to the "Resource" tab from an existing container and edit its disk, there is also such an option.
Seems more like Proxmox VE 4.0 not 4.1, can you exec a
Code:
apt-get update
apt-get dist-upgrade
The current pve-manager version in the enterprise repo is 4.1-22 (and in no subscription it should be that or newer).
Quota support was not added that long ago.