There is a fifth handshake, and it may not be very well known, but Robert and I were startled that both of us were aware of it.
Right after you've deployed 'The Firm Shake' with someone you'd really like to get to know better , you use the 'flip the bird' finger and lightly scratch the palm of the other person.
This is code for 'I'd really like to do naughty things to you'. Yes, it's cut to the chase and you may be considered a jerk by those in the know, but it could just as easily lead to fun and games. Even if nothing comes out of it, there's a good chance you've delivered a rush to someone who didn't expect it. And of course, if your 'prey' has no idea what it means, they certainly won't forget your face and may be inclined to spend some time figuring you out.