The first Olympic Games were held in Japan in 1964. This was the summer Olympics in Tokyo.
The second was held in Sapporo in 1972 and the third was held in Nagano in 1998. The Olympics in Sapporo and Nagano were both winter Olympics.
Tokyo2020 summer Olympic is the Olympic that follows these Olympics.
So,Tokyo2020 is the fourth Olympics in Japan in summer and winter, and the second Olympics in Japan in summer alone.