The board and setup
The board is 4×4, with a farm to either side that holds captured pieces. A tile's owner is shown by its facing: the first player's tiles point up the board, the second player's point down.
The pieces
Every piece moves one square per turn, in the directions printed on its tile. On reaching the far row, each non-royal piece evolves, flipping to its evolved side. The pairs below show the base piece, then its evolved form, with a dot on every square each can reach (forward is up).
Carp → Koi. The Carp steps one square straight forward, a pawn. It evolves into a Koi, which moves as a silver from shogi.
Tapir → Baku. The Tapir steps forward or to a forward diagonal. It evolves into a Baku, a silver.
Raccoon-dog → Tanuki. The Raccoon-dog steps one diagonal. It evolves into a Tanuki, a silver.
Fox → Kitsune. The Fox steps one orthogonal. It evolves into a Kitsune, which moves as a gold from shogi.
Crane / Pheasant. The royal steps one square in any of the eight directions, a king. The two royals differ only in theme. It never evolves, and capturing it ends the game.
Jumping over a friendly piece
A piece can leap over a friendly piece. If an ally sits on the next square in a direction the piece moves, the piece jumps it and lands on the square just beyond, empty or capturing an enemy there. It works in any direction the piece itself moves: straight for a Carp, on the diagonal for a Raccoon-dog, any of the eight for the royal.
Capturing, farms, and drops
Move onto an enemy to capture it; it switches sides into your farm, reverting to its base form if it was evolved.
Instead of moving, drop a piece from your farm onto any empty square, except those on the far row (the opponent's back rank).
Winning
Capturing the royal is the only way to win. No check, no checkmate: the game ends the moment a royal is taken.
There is no stalemate. Because moving the king into capture range is legal, a lack of safe moves never ends the game: you simply make the unsafe move and play on until a king is taken. A side with no legal move at all, boxed in with nothing to drop, loses rather than draws.
Repetition and draws
The original rules address neither repetition nor a move-count limit. Our convention fills the gap: a position reached three times is an automatic draw. That rule is ours, not Oca's, and changes none of the rules above.
A sample game
Fairy-Stockfish self-play on the friendly-jump engine (this site's patched build). White wins in 73 plies; the mating move is itself a friendly jump.
Source and license
Shogi4 and its tile art are by Oca Studios, which released its whole "Four" series into the public domain. The BoardGameGeek entry is a catalog reference.
We recovered the exact rules from Oca's official Shogi4 app, decompiling it to read the move logic directly: the friendly-jump geometry, the single drop ban, and king-capture as the sole win all come from there. Oca's public rules page and starting-position graphic (now reachable only through the Internet Archive, since the live site is down) corroborate the board and the basic moves.
Playing Shogi4
Shogi4 isn't playable on the site yet; for now this page is the rules reference. Browse the rest of the rules, or compare it with the chess and xiangqi primers.