HUNDREDS of Japanese finance industry employees walked off their jobs yesterday to demand higher bonuses and shorter working hours and protest improper practices by the Big Four brokerage houses.About 1,300 union workers, many from the Tokyo Stock Exchange, marched to the stock exchange and the headquarters of Nomura Securities, the world's biggest brokerage. They then marched on the Ministry of Finance as well as on the Japan Securities Dealers Association. "We are outraged to see how the management is trying to survive the current stock market slump at the cost of workers' benefits," said Isamu Oowaku, who organized the five-hour strike and demonstration. The exchange union, with 500 members between 20 and 58 years of age, is demanding a winter bonus for each employee of 4 1/2 months' pay. The average member is 29 years old and would be entitled to $9,700. Management has offered 2 1/2 months' pay, which would give the average worker $5,260. Mr. Oowaku said management had put too much emphasis on pursuit of profit, which led to improperly compensating investment losses for elite clients and links with crime syndicates. The Big Four - Nomura, Daiwa Securities Co. Ltd., Nikko Securities Co. Ltd., and Yamaichi Securities Co. Ltd. - have admitted improperly compensating clients for losses worth $334 million in the year ending March 31, 1991, and for losses worth $987 million in the 30 months before March 31, 1990.