4 nights? You can go almost anywhere in Europe for that! No, it's not enough time to really SEE London or Paris, but it's great for seeing a bit of theater, ballet or opera, having a great meal or two, catching a special exhibition or revisiting a favorite museum, and wandering. (I frequently take 3-night trips to those cities and do just that). And there's a host of possibilities elsewhere--spend 2 days in Oslo and 2 in Bergen (I'd do that when you could take a fjord trip), go to Prague or Budapest or Venice or Florence or Madrid or Barcelona, do some combination of Vienna, Salzburg, Innsbruck, or go to Berlin, or do a combination of Edinburgh and Innisfree (at the right time of year), or spend one night in London and grab a low-cost flight anywhere the fancy strikes.