TOKYO — Masayoshi Son makes big bets. Over nearly four decades, the chief executive of SoftBank Group of Japan has spent vast amounts of money to build an investment empire that spans countries and industries and that has increasingly shaken up the technology world.

On Wednesday, Mr. Son , 62, defended that legacy in the face of his biggest setback in years. SoftBank reported that it had taken a nearly $4.6 billion hit from its investment in WeWork, the troubled office-space company that has come to symbolize the excesses of start-up culture.

“In the case of WeWork, I made a mistake,” he told investors at a news conference in Tokyo. “I won’t make any excuses. It was a very harsh lesson.”

Still, Mr. Son vowed to press on with his grand plan to make major bets on ambitious companies and their founders, hoping to find visionaries whose companies will go on to dominate entire industries.