The Hiro H4H (or Hiro Navy Type 91 Flying Boat) was a 1930s Japanese bomber or reconnaissance monoplane flying boat designed and built by the Hiro Naval Arsenal for the Imperial Japanese Navy.

First appearing in 1931, the H4H1 was a twin-engined, high-wing monoplane flying-boat.

Powered by two 500 hp (597 kW) Hiro 91-1 engines strut-mounted above the wing, it was produced by the Kawanishi company and entered service in 1933.

[1] An improved version of the design, the H4H2, followed into production two years later.

The H4H2 has re-designed twin fins and rudders and was powered by two 800 hp (597 kW) Myojo radial engines.