Most of?
Kawasaki is the only one that has traditionally been called "Heavy Industries", being in the ship building and airdraft industries.
Toyota and Nissan have always been car makers, dating back to well before WWII.
Suzuki was a company that built looms, and Yamaha built pianos.
Honda is the new company out of the four, and is the only one that actually started out making motorcycles.
Kawasaki, Yamaha and Suzuki were trying to rebuild war-ravaged factories, and began making motorcycles to help feed the post-war demand for cheap transportation in Japan, which is also how Honda got started.
Toyota and Nissan resumed making what they knew best; cars and trucks.