How to plot 40GB hspice output?
You can open up sections of a large text file in matlab, you may be able to sub-sample the text data and plot it out. I would propose a different solution. I would reduce the file size first. 

Depending on how long the simulation takes. I would only output one signal per file. So if you have hspice recording multiple signals in your output, only do one at a time and store the file and run the simulation multiple times.

Ask yourself if you really need the resolution of the simulation, and reduce it to something more manageable.

I know LTspice will let you set a time for the recording of data after the simulation start time, hspice may also have something like this.