Here's a little more info on your coin. While kan ei tsu ho coins were produced for over 230 years in japan, i can pin point the exact date of this one.
On the 1st day of the 5th month of 5th year of Meiwa (15th June, 1768) a larger Kanei Tsuho cash, made from brass with what are called 波 nami or waves on the reverse, and having a value of 4 mon, instead of the smaller 1 mon was put into circulation.
The first version had a 21 wave reverse, but the next year the design was changed to 11 waves. (your coin)
A second batch of these coins was made in 1821-25 during the Bunsei era using a copper/lead
alloy, and a third in 1857-59 in the Ansei era with a higher lead content. Collectors refer to these as
明和Meiwa, 文政Bunsei and 安政Ansei coins.
The color of the coins is the main criterion for attributing them to periods, because the calligraphic
differences are very slight.
the Meiwa coins (1768, 1769) were 68% copper, 24% zinc and 8% tin. these coins are "brassy" in color.
The Bunsei era coins (1821-1825) were 75% copper, 15% zinc and 10% lead. These coins are "reddish" in color.
The Ansei era coins (1857-1859) were 65% copper, 15% zinc and 20% lead. These coins are "Dark".
Your coin is "brassy" and is the 11 wave version. Therefore it was minted in the year 1769. There were 157,425,360 of these coins minted.