chessie,
I think it has already been mentioned but, if not, a couple of things to be aware of. Using the cardboard strip method with plaster cloth has one downfall - the cloth will "sag" in between the cardboard strips no matter how hard you try to keep it taught, or even reasonably taught. A better, and quicker method (I think) is to use scrunched up news paper to form your mountain/hill then use the plaster over the top of that. You could even do a paper mache type of thing.
How much cloth will you need? Lots, even for a small area. To obtain a "reasonably solid" surface, you will need to have at least two (2) layers of the cloth, that is even what the manufacturers recommend. When I did my first layout I used the newspaper/plaster cloth method and found I had to do three layers of the cloth to obtain any sort of good base with which to work with.
If memory serves me correctly, Plaster Cloth is about 8" wide and in rolls 10' long. Your 8 X 4 layout will need about 10 rolls to cover it fully with each strip of the cloth overlapping the previous by 50% giving you basically 2 layers. If you are using the cloth JUST to create a mountain, then I would strongly recommend you use a minimum of 3 layers of the stuff. How you will "fill the gaps" (where it sags) is another matter though.
One other small problem I have found using plaster cloth as well, the amount of actual "plaster" in the cloth is minimal and does not fully cover the cloth itself. What I mean by that is, even when you have wetted the cloth, layed it all out and "smoothed it down", you will see a lot of little "holes" where the plaster hasn't covered properly. Depending on the make of the cloth, that may be apparent even after two layers. This may not be an issue (of course) as the cloth will be painted and covered with ground covers etc, so it may not matter with regards aesthetics; however, where the plaster does not cover means that/those spots will be weaker.
I am not trying to put you off using Plaster Cloth as it is a good start if it has a solid base to sit on, such as foam or even a wire mesh etc; however, I don't think it is a great method if the cloth is going to be relied upon as the base on its own as well as some of the pitfalls with it, not the least being how much you will/may need.
So, my recommendation would be (if using plaster cloth) to use the scrunched up newspaper method to form your mountain/hill then cover that with the cloth rather than the cardboard board strip and cloth method.