Reliance Industries Chairman Mukesh Ambani has overtaken Europe's wealthiest man Bernard Arnault to become the world's fourth-richest person, according to the Bloomberg Billionaires Index. Ambani is now worth $80.6 billion, after gaining $22 billion this year. The 63-year-old trails only Facebook CEO Mark Zuckerberg, Microsoft Co-Founder Bill Gates and Amazon CEO Jeff Bezos on the Bloomberg index.