The download is a bit heavy for South Africa.
I'd like to pay for an additional 4 copies, but let the other guys copy my game. Is it easy to figure out what CD Key the additional copies get, and then update each install with its own CD Key?
TIA
Sheldon.

I don't exactly know how CD keys are given for this game (I've installed it from CD, as I've only recently acquired it on GOG), but why don't you do the following: buy the game for the other three people, using their own accounts (create them for them, if they don't already have their own accounts), so that they can have their own CD keys, and then give your copy to them. I suppose this is an acceptable solution.
Figuring CD-keys out, however, may be a not-exactly-legal procedure, even if you apply the new keys to legal copies. At least that's what I know.

I believe Dristvann is on the right track, have each friend create an account and purchase PK.
After purchasing the game, log in and click (top of page) Support, Game Support, at the bottom of the Game Support page, click Contact Us [url=]http://www.gog.com/en/support/contact[/url] , select the first option of Technical Issues with games and request a CD key for your game.
I received my CD keys in a couple of days following the above procedure. You could contact GOG prior to this but I believe it would meet all the requirements for getting a CD key for everyone without having to download the game multiple times. Good luck and enjoy PK !
Edit: Each friend would need to do this for their account. Prior to doing the above, contact GOG to confirm that it meets their requirements. It is the way I got my CD keys but I am not certain it is the correct procedure for what you want to do.

The only way he can do this legally is for his friends to create their own accounts and use them to get the license key. You can't buy multiple copies of a game for yourself, you can only buy it once. Your only option for buying it again is to "gift" the purchase to another GOG member, so unless his friends have GOG accounts, he can't buy four copies of the game in the first place.