[Cryptography] Smallest 4-bit S-box?

[Ryan Carboni]

4-bit s-box [4, 8, 14, 2, 1, 13, 7, 15, 10, 12, 0, 9, 6, 3, 5, 11]
The method in generating the S-box is encrypting the four bits through
the following function 7 times:
Bit1 = ~((Bit2 & Bit3) | Bit1) ^ Bit4
Bit2 = Bit1 ^ Bit3
Bit3 = Bit2
Bit4 = Bit3

Essentially it's a Feistel network using an AND-OR-INVERT gate to
achieve nonlinearity, and uses a single xor to prevent it from being a
permutation of the input when at least three plaintext bits are one.
The S-box as is would use a minimal number of transistors.

If used in a cipher, it would ideally be used in a Type 1 Feistel
network similar to XXTEA.