By popularity, this is the hand signal

[Wolff <wolff_b@ix.netcom.com>]

We (whoever wants to) are going to use 4 fingers up with thumb tucked
across palm with palm facing recipient. Optionally you can follow with
the "OK" sign. so, kids, with a little practice you can go from 4
fingers to OK in a neat one two motion. Don't try this at home. (only in
your Q)  ;)

Sorry, but the middle finger up solute that some suggested thankfully
only came in second in popularity.  :)