I would like to add a wash machine to the basement. Near the area of the basement where I'll be adding it is a 4 in drain coming from an upstairs toilet. Can I tap the new drain into the toilet drain?
You can tap in to the existing stack to drain, the only issue (like bcworkz said above) is you need proper venting. If you were to tap in to that stack without a vent, you could potential pull the water out of any traps above (via suction).