Boil 'em in oil.
Ooops, sorry, forgot to switch off 'curmudgeon' mode...
give them a copy of the Motley Fool UK investment guide I reckon, seeing as Terry Pratchett is getting a bit samey these days.
Failing that anything with a good laugh in it will do
I think hitting 4000 members will definately be a cause for celebration and at the current rate could just be a few months away. Certainly a free book, or even better, a T2W tshirt printed with our new logo (should have them ready by then - long overdue now!).