The sun is directly overhead (at the zenith) when it's at a 90-degree angle relative to the observer. This phenomenon typically occurs in the tropics, between the Tropic of Cancer (23.5 degrees North) and the Tropic of Capricorn (23.5 degrees South). These are the maximum latitudes at which the sun can appear directly overhead.
Japan is located between about 24 and 46 degrees north latitude. This means it is generally too far north for the sun to ever be directly overhead.
On July 4th, the sun is still moving northward from the Tropic of Cancer (which it reaches at the summer solstice, around June 21st). As a result, even in the southernmost parts of Japan, the sun will not be directly overhead, though it will be relatively high in the sky. In more northern areas, the angle will be even more pronounced.
If you want to know more precise details, such as the exact angle and time for a specific location in Japan on July 4, you'd need to consult a detailed almanac or a sun position calculator that takes into account the exact coordinates and date.
---
It no longer hallucinates an incorrect answer, though I don't have the knowledge to know if it's correct in saying that there isn't one.