If you just found out you're 4 weeks pregnant will drinking 1 wine cooler about 3 weeks ago affect the baby?

Highly unlikely as that would have been before you even got pregnant. There's not a whole lot of alcohol in a wine cooler so it should be ok, but now that you know you are pregnant, you should avoid alcohol. When you are told you are 4 weeks pregnant you actually got pregnant 2 weeks before.