Microshogi is a Shogi variant played on a 4×5 board with hybrid pieces -- non-standard pairings of standard units:
Bishop / Tokin
Gold General / Rook
Silver General / Lance
Pawn / Knight
King
A piece is promoted (flipped) whenever it captures another piece. This is mandatory even when the flip side is a weaker unit.
Also unlike 9×9 Shogi, captured pieces held in reserve can be dropped without restriction:
A piece can be dropped with either side up.
A piece can be dropped onto a rank that prohibits it from any further move.
A pawn can be dropped onto the same column as another allied pawn.
A pawn can be dropped to give immediate checkmate.
Microshogi is believed to have been designed by Shogi Grand Master Yasuharu Oyama (1923-1992).