No.4 is dated from 1736-1796, and No. 5 is dated from 1821-1851
The first, no.4, is the reign of Emperor Qianlong; no.5 is Emperor Daoguang.
The Qianlong emperor is considered one of the greatest in Imperial Chinese history. Upon assuming the throne, he said that if he was still alive after 60 years, he would abdicate in favour of a younger successor; in the end he was still alive, and kept his promise. He collected works of art obsessively; had an Italian Jesuit missionary (who adapted to China to such a degree he had a Chinese name and painted in Chinese style) working in the court--one of his projects was to design a series of palaces in Western styles. While European rulers often had a single room in their palace decorated in Japanese or Chinese, style, Qianlong had entire buildings scattered among the imperial grounds.
There was a thread here where IIRC doug222usa
posted a website showing a coin-faking operation. These places can make a near-perfect die. They do slip up though--for early silver US Trade Dollars, the year they stamp on the coins pre-dates a bit the year the coins were first minted.
Sometimes the coins are slightly off in weight, so a way to tell.
For ancient Chinese copper coins, even if they're original they might not be worth much. Even hundreds of years ago the population of China was in the tens of millions. Copper coins were for most places in the world the everyday money. Look at the Roman coin hoards found in the UK; similar can be found in China. There's no shortage of supply in some cases, so something 2000 years old can be had for a few dollars.
The coins have a hole in the centre so they could be carried on strings (also done in PNG, but in that case it was because the natives had no wallet or pockets--or no clothes at all to carry money in
There's all kinds of tricks that forgers utilize to age bronze (copper alloy) "antiques" to give them the green patina. The lazy or stupid just paint green on.