You start off with [a b c d] Push in 2 empty stack frames OP_0 OP_0 [a b c d 0 0] Rotate bottom pair to top OP_2ROT [c d 0 0 a b] Move top frame to alt stack OP_TOALTSTACK [c d 0 0 a] [b] Nip away the empty frames, OP_NIP OP_NIP [c d a] [b] Put back last element from altstack, OP_FROMALTSTACK [c d a b] Ta-daa!! Entire script: OP_0 OP_0 OP_2ROT OP_TOALTSTACK OP_NIP OP_NIP OP_FROMALTSTACK