I don't have a friend at cisco but I have friends in other large name companies and usually when they force shut down (esp. around the holidays) It comes out of the employees vacation days.

My friend works at Cisco and told me about this last week. They are indeed taking the days out of their employees' vacation days. If they don't have any vacation days, they will be on unpaid leave for the week.

This type of thing has also happened to other friends of mine. It's a cost-saving maneuver and also helps lower the companies' liabilities due to having so many vacation days on the books (since the company has to pay you money for each vacation day you have once you leave).

Thought you were supposed to be able to "borrow" from future PTO.

If what you say is true, this measure doesn't seem to make any sense.

If their employees cost more than what they produce every day, doing nothing for 4 days is similar to closing your eyes and wishing the problem away.

I think that's a little bit of an oversimplification. Their two basic options are a shutdown or layoffs. I'm guessing they are doing the shutdown and hopes that sales rebound and they can avoid a round of layoffs. There is a cost to laying off people as well.

If they're losing money every day, closing down for 4 days makes sense.

Yes, as much as closing down for the whole year.

While that's an oversimplification to be sure, what you just described is also known as "returning the money to the stockholders".