Four days is more than enough for London. You may be able to take the bus tour.. however, it's a must to ride the London Eye, and the river cruise (you can book them together). There are other associated attractions, but I am not sure they are very nice. The bus tour should give you an idea about the other important points, but I would say "British museum", "national gallery museum", "St. Paul's Cathedral", and Parliament square leading to Trafalgar square and passing by Downing Street where the prime minister resides. Some people are interested in Hyde Park and Buckingham Palace, so please ask your hotel about the time of the change of guards.. that's the nicest thing about them.. When are you coming (dates)?!!!