If I want to make 4 cups of coffee (in the pot), which is actually 2.5 cups if I put it in a regular liquid measuring cup, how much coffee should I start with?
The answer is that it will depend on your personal preference for the strength of your coffee, the beans themselves, the type of brewing, and so on. Many coffee bean bags have a reference on the side of them.
The standard guideline for most medium roast coffee bean bags is 2 tablespoons per 6 fl. oz. (1 cup in your liquid measuring cup).
The Specialty Coffee Association of America has some interesting info on this, per this link.
Serious Eats has just published an article explaining why measuring coffee by weight is much more accurate—and it is the same measurement whether you weight the beans whole or ground.
The bottom line is weight is simply much more accurate, as it removes variables of size, packing, consistency and so on. Consistent measurement helps ensure consistent results.
The recommended amount of coffee in the article, by weight, is approximately 30 grams (1 ounce) per 12-16 oz of water. So, in the example from the original question, for four "cups" (at 6 oz per cup) you would want 60 grams (2 ounces) of coffee. Note that this recommendation is higher than the 10 grams/cup that is indicated in the original answer this this question. In the end, each person will need to determine the ratio that is ideal for their taste.
In fact, they recommend weighing the water as well, although in my personal opinion, this is less critical as over say a 32 ounce pot (6 "cups" or three of my mugs, the smallest I personally consider worth brewing), the accumulated error is much smaller.