Basil is not a plant that can survive a winter in Z4. I sugggect that you give us a photo if possible. Smell isn't the only thing that can help. Maybe its mint, oregano, or some kind of similar herb. Basil isn't very hardy after August in the Z4 areas.

Could be anything, but possibly amaranth or perilla, both of which can have purplish leaves, and both of which can easily become weeds. The seeds spread far and wide.

Get it professionally identified. NEVER, never use a plant unless you are 100% sure what it is. Too make poisonous look-alikes out there - Nature isn't always friendly!

A good herb nursery, your botanical gardens, your local agricultural authority should all be able to help. Take a cutting, preferably with flowers and/or fruit, and make sure it's as fresh as it can possibly be when they see it. Otherwise they'll probably simply call it 'a dead plant'. Fair enough, too!

A week ago, I took the plant to a local nursery and they identified it as something in the Basil Family but were unsure of what specific plant it was untii they could see the buds. It has flowers now and it looks like Thai basil.