This is a known bug with Medtech where the disk isn't actually out of space, but that the free disk space is at a value of a multiple of 4GB. (4GB, 8GB, 12GB, 16GB free etc)
The only way to resolve this error is to exit Medtech by going to File > Exit and reopening it. If the problem persists please contact the help desk on 0800 367 864 or at Helpdesk@tmg.net.nz