I have the same problem with my HD4
I have contacted Nextwave about the issue and they don't seem to be concerned, telling me much of the same, it's normall for the weight, put a piece of wood under it, get a battery backup, if they had safety in mind they might offer a Z axis power off stepper motor with a electromagnetic brake which I have asked them about and if they can't make one up can they give me the specs on the Z axis motor that is included with the HD4 still haven't got an answer, if anyone has the specs on this motor please post it, some say it's a Nema 23, Nema 23s only have four wires this has an extra white wire? Any help would be appreciated
The reason some stay up and others don't is that the lead screw and the motor is not lined up, in other words it is in a bind, watch the bottom of the skrew as you operate the Z axis with the speed set to slow you will see that if it doesn't wobble your spindle will probably drop when power is cut, if it wobbles yours wont!