Surprisingly few billionaires in any of the 4 countries obtained their wealth through political connections. Founding a company seems to be still the path of great wealth even in Japan these days. Hong Kong is a financial centre so the large number of billionaires in its financial sector is no surprise.
Japan has gone first to be last having been just overtaken in the last year or two by South Korea on a per capita real GDP basis, PPP. The Lost Decade certainly has taken its toll on Japanese relative prosperity. Singapore overtook Japan in the 1970s – a testament to the Singapore miracle. Hong Kong too overtook Japan on a purchasing power basis in the mid-1990s followed not long after by Taiwan. Singapore is seriously rich.
Source: The Conference Board. 2015. The Conference Board Total Economy Database™, May 2015, http://www.conference-board.org/data/economydatabase/