Here is what was found in layer 4 - a crown cap closures on soda pop bottles, which dates from after 1892 in the US. This means that the date for Canada would be slightly later, because it would take some time for the technology to travel north to Canada. Although not as popular now as they were about 10 years ago (on soda pop bottles), crown cap closures are still used on some bottles today.
The coin has a 1995 stamped on it indicating the year it was made.
Plastic items were commercially available to the public after World War II, sometime in the late 1940's to 1950's. This means that this plastic button probably dates to after the 1940's or 1950's.
This gives us a date of post 1995 for the items found in layer 4.