My main stack/vent which penetrates my roof is 4" plastic. It comes down vertical until it gets to the first floor ceiling where is turns 90 deg and runs horizontal to an outside wall. About 3 feet from this 90 deg elbow there is a "Y" connection for a toilet. I'm not sure but I assume at the point of the toilet connection this main stack stops being a vent and starts being a drain. I have a 2" vent from floors below which I would like to connect to the main vent or stack. Can I connect this pipe to the horizontal 4" pipe between the toilet and the 90 deg elbow or do I need to connect to the stack on the other side of the 90 deg elbow where it is vertical?

My main stack/vent which penetrates my roof is 4" plastic. It comes down vertical until it gets to the first floor ceiling where is turns 90 deg and runs horizontal to an outside wall. About 3 feet from this 90 deg elbow there is a "Y" connection for a toilet. I'm not sure but I assume at the point of the toilet connection this main stack stops being a vent and starts being a drain. I have a 2" vent from floors below which I would like to connect to the main vent or stack. Can I connect this pipe to the horizontal 4" pipe between the toilet and the 90 deg elbow or do I need to connect to the stack on the other side of the 90 deg elbow where it is vertical?
You need to be at least six inches above the flood rim of the water closet before you can combine your 2" vent into the existing vent.

The flood rim is the top of the water closet bowl, by the way.

Is there any reason why two 2" vent pipes can't be combined? I have a 2" vent from my basement and I need another 2" vent at the main floor for my kitchen. I would like to combine the two into one which will then continue on to connect to the main 4" vent to the house located above.

Web,

You are correct when you say, "I'm not sure but I assume at the point of the toilet connection this main stack stops being a vent and starts being a drain."
It stops being a vent stack and starts being a stack vent.
And growler's correct with his advice. What you're planning is called a "revent" and a revent MUST be connected at least 6 inches above
The flood rim of the highest fixture.

As for combining two 2" vents it all boils down to how many fixture units will be on the vent. Code allows 20 fixture units on a 2" vent. If your basement group consists of a toilet, lavatory, shower, and a washer that would give you nine fixture units. Add a kitchen with a disposal and you have another three units. They add up to 12 fixture units so merge the two vents. Just remenber growlers advice and revent back 6" above the rim of the toilet. Good luck, Tom