So, where’s good for about 4 days in Europe?
Unless you posted your OP from North America, or somewhere else that isn’t Europe, you already are in Europe. I assume you mean the Continent.
Now that that’s out of the way, I really think it’s helpful to determine if your and partner’s interest’s are more northern European or southern, in the sense that visiting a city south of the mountains (Lisbon/Barcelona/Malaga/Rome) is going to give you a different experience to one to the north (Bruges/Paris/Berlin/Copenhagen/Prague). And not just because of temperature.
In any case, my vote would be for Berlin. It’s fantastic: vibrant, historic, modern, full of beer, sausages, schnitzel, museums…