Knight Hop

An $8 \times 8$ chessboard is given. Each square is identified by an ordered pair of integers, where each coordinate ranges from $1$ to $8$. For example, in the diagram below piece A is at position $(2, 2)$ and piece B is at position $(4, 3)$.

A knight moves in an "L" pattern: it can leap over other pieces to one of up to eight squares. In the diagram below, K marks the knight's current square and the numbers $1$ through $8$ mark the squares it may move to.

Given the knight's starting square and a target square, compute the minimum number of moves needed to move the knight from the start to the target. The knight may never leave the board.

Four integers, each between $1$ and $8$. The first two give the knight's starting position; the last two give the target position.

Output the minimum number of moves (a non-negative integer) required to move the knight from the start to the target. The knight must not leave the board at any point during the sequence of moves.