[ddfs123]

I've consider it to be 4 handshakes but the middle 2 are merged.

[ay]

There is a scenario called “simultaneous open”, which has 4 packets handshake and is a wonderful source of corner cases and debugging. Mostly doesn’t happen these days, but is possible by standard and is explicitly described.

[winternewt]

It has been used for NAT traversal when both parties are behind NAT, but sadly it doesn't work with all routers.